Was Crazy Horse's mother a Brulé ? Not to Crazy Horse 3. But perhaps to Crazy Horse 2 or Crazy Horse 1.
Earlier on this website Kingsley Bray gave an overview of the sources that refer to Crazy Horse's mother as Mnikĥówožu Lakĥóta (Miniconju Lakota) . That is Crazy Horse 3 (Tĥašuŋke Witkó 3) (aka Among The Trees (Čháŋ Óhaŋ), Light Hair (Žiží), Jiji (Žiží), Curly Hair (Pĥehíŋ Yuĥáĥa), His Horse Stands in Sight (Tĥašuŋke Tĥaŋíŋyaŋ)) In short, 4 contemporaries to Crazy Horse 3 said his mother was a Mnikĥówožu Lakĥóta . In addition, several modern Lakĥótas confirm that Crazy Horse's 3 mother was a Mnikĥówožu Lakĥóta [1, 2, 3]. Consistently, those who said her name all agreed upon the name Rattle Blanket Woman (Rattling Blanket Woman) [1, 2, 3]. So based on all these various sources, it is well established that Crazy Horse's 3 mother was the Mnikĥówožu Lakĥóta Rattle Blanket Woman (Rattling Blanket Woman). Therefore Crazy Horse's 3 mother was not a Sičháŋğu Lakĥóta (Brulé Lakota).
The mother of Rattle Blanket Woman and therefore Crazy Horse's 3 maternal grandmother was the Hunkpatila Lakĥóta White Cow Woman (aka Iron Cane) [4, 5]. White Cow Woman (aka Iron Cane) was a wife of the Mnikĥówožu Lakĥóta Black Buffalo (aka Black Bull & Crippled Warbonnet) [4, 5].
However, it is not known (at least to non-Lakĥótas) who was the mother of Crazy Horse 2 (aka Waglúla (Worm) = father of Crazy Horse 3). That woman was Crazy Horse's 3 paternal grandmother, Crazy Horse's 2 mother and Crazy Horse's 1 (aka Makes the Song) wife. She might have been a Sičháŋğu (Brulé) Lakĥóta. That would mean a Sičháŋğu (Brulé) Lakĥóta woman married to the Oglála Lakĥóta (Oglala Lakota) Makes the Song. If so, that could perhaps put a new light on the hunka relationship between Crazy Horse's 3 and the Sičháŋğu (Brulé) Lakĥóta Iron Shell. Where some relation could have been between Iron Shell and Crazy Horse's 3 paternal grandmother.
Mother of Crazy Horse 1 (aka Makes the Song) was Red Eagle Woman and she was a wife of Black Elk 1 [4, 5]. Red Eagle Woman was Crazy Horse's 3 paternal great-grandmother. She might have been a Sičháŋğu (Brulé) Lakĥóta, but she lived in an Oglála camp with her husband.
So there could be something in the statement that Crazy Horse's mother was a Sičháŋğu (Brulé) Lakĥóta. Although it does not apply to the famous Crazy Horse (Crazy Horse 3).
Yellowhair1850: What is your source that Crazy Horse's mother was Brulé ? Is it a family oral tradition ? Or is it from the book Red Cloud's Folk by George Hyde on page 298 ?
Good remark Grahame. But most likely, Sandoz got this "Spotted Tail's sister story" from Hyde who had published it 5 years earlier in his Red Cloud's Folk book.
It is interesting to notice for how long time this "Spotted Tail's sister story" has been around. It started with Hyde in 1937  and has been repeated uncritically over and over for about 70 years - almost a century ! No writer gives a reference to any source regarding the affirmation that Spotted Tail's sister was Crazy Horse's 3 mother/stepmother ! Neither a testimony of a contemporary of Crazy Horse 3 nor a modern Lakota oral tradition.
First only 1 sister of Spotted Tail was in the story. Later it became 2 sisters. First the Spotted Tail's sister was supposed to be Crazy Horse's 3 mother. Then writers turned her into his stepmother. Perhaps after some writer had found out who was Crazy Horse's 3 real mother, and others writers followed the example without critical thinking. It was Hyde (1937 & 1961) and Sandoz (1942) who wrote that Spotted Tail's sister was a mother of Crazy Horse 3. Other writers said she was his stepmother, as detailed below.
In 1937 Hyde wrote ; "Crazy Horse was supposed to be about 33 years old when he died. His father was an Oglala named Crazy Horse and his mother was Spotted Tail's sister."
In 1942 Sandoz wrote [2, 3]; a) "his mother who had died was a Brule and the sister of Spotted Tail, as was his second mother, in the custom of the Lakotas."  b) "for it was well known that his mother was the sister of Spotted Tail, the Brule." 
In 1961 Hyde wrote ; "Two of his (meaning Spotted Tail) sisters were married to an Oglala Sioux medicene man named Crazy Horse, and the famous Crazy Horse of Custer battle fame was the son of Spotted Tail's sister."
In 1985 Hardorff wrote ; "This second marriage was with two of Spotted Tail's sisters, Brules, one of whom bore him a son circa 1846. Known as Little Hawk".
In 1998 Hardorff wrote ; "Worm married two sisters of Spotted Tail, one of whom gave him a son named Little Hawk".
In 2000 Sajna wrote ; "he (meaning Worm) eventually married two sisters, siblings of the Brulé Lakota chief Spotted Tail."
In 2004 Marshall III wrote ; "They were sisters, quiet and polite as they walked into the circle of their new life as the wives of Crazy Horse and the mothers of his daughter and son. They were the younger sisters of a man renowned among his own Sicangu people, one whose name was spoken often in reference to courage and leadership: Spotted Tail."
In 2004 Cozzens wrote ; "The woman with Worm was not Crazy Horse's mother, Rattle Blanket Woman, who had committed suicide in 1844, but rather the old man's second wife, a sister of Spotted Tail."
Finally in the year 2006, this was sorted out. When both the Clown family published their DVD  and Kingsley Bray published his book . Then it became clear it is WRONG that Spotted Tail's sister was Crazy Horse's 3 mother or stepmother.
I think what lies behind the various versions of "Spotted Tail's sister(s) story" is a mixture of 4 parts of facts: 1. Crazy Horse's 3 mother was the Mnikĥówožu Rattle Blanket Woman, who commited a suicide in 1845 [10, 11]. 2. Worm married the Mnikĥówožu daughters of Corn in 1844 [10, 11]. These sisters became Crazy Horse's 3 stepmothers. 3. Mnikĥówožu sisters of Rattle Blanket Woman came to Worm's Oglála camp in 1845 and took part in their sister's role in raising up Crazy Horse 3 [10, 11]. These sisters became Crazy Horse's 3 fostermothers. 4. Crazy Horse's 3 mother-in-law was a "sister" of the Brulé Spotted Tail . Because Black Shawl's mother, the Brulé Red Elk Woman, was 1 of 2 daughters of the Brulé Old Spotted Tail (Spotted Tail 1) who adopted the famous Spotted Tail (Spotted Tail 2) . The Brulé Red Elk Woman married the Oyuhpe Oglála Red Feather. Together they had the daughter Black Shawl, who Crazy Horse's 3 married in 1870 . Upon the marriage of Crazy Horse 3 and Black Shawl, Worm and Red Elk Woman (Spotted Tail's sister) became related as parents of "the groom and the bride".
The various stories about Crazy Horse's mother/stepmother being a Spotted Tail's sister are a different mix of the above mentioned facts. With the 4th part above as the most influental in the "Spotted Tail's sister story". The first published version of the Spotted Tail's sister story" was published in 1937. Which is many decades and up to a century after the happening of the events which the story describes. Giving enough time for a biased story. Originally, this misunderstanding was perhaps a pure translational error. Or because of the difference in the meaning of the word mother between 2 cultures. With the meaning of the word mother much more "open" in the Lakota culture. In Lakota culture it is an important factor to make as much and strong bonds between people as possible, as pointed out somewhere (on this website ?) by Kingsley Bray (and perhaps others also). For the better good for the society and as a form of a welfare system in times of difficulties.
A further complicating factor behind this "Spotted Tail's sister story" is the possibility that Crazy Horse's 3 paternal grandmother[/u] and/or paternal great-grandmother[/u] was a Brulé. As described above in reply #2.
References: 1. Red Cloud's Folk by George E. Hyde (1937), page 298. 2. Crazy Horse by Mari Sandoz (1942), page 18. 3. Crazy Horse by Mari Sandoz (1942), page 42. 4. Spotted Tail's Folk by George E. Hyde (1961), page 15. 5. The Oglala Lakota Crazy Horse by Richard G. Hardorff (1985), page 32. 6. The Death of Crazy Horse by Richard G. Hardorff (1998, 2001), page 56. 7. Crazy Horse by Mike Sajna (2000, 2005), page 27. 8. The Journey of Crazy Horse by Joseph M. Marshall III (2004), page 11. 9. Eyewitness to the Indian Wars 1865-1890 - The Long War for the Northern Plains (2004) by Peter Cozzens page 672. 10. DVD disc: The Authorized Biography of Crazy Horse and His Family, Part One - Creation, Spirituality and The Family Tree, made by Reel Contact in association with Tashunke Witko Tiospaye (2006) (www.reelcontact.com) 11. Crazy Horse - A Lakota Life by Kingsley M. Bray (2006) 12. amertribes.proboards.com/index.cgi?board=teton&action=display&thread=118
I don't know if, I can help but, will try. I reviewed some of my older notes tonight and made several new notes. In doing so, I worked on a new chart. The chart starts with some information about; Makes The Song (aka), White Rabbit and for now, ends with my relative, Wakinyan (aka), Thunder (aka), Thunderbird (aka), Dave Yakama Chief. Other relatives included in this are: Black Fox, Iron Cedar Woman, Shoots For Them, Thunderbird, Good Jumper (aka), Woodpecker (aka), Chasing Jumper, Kicking Bear and many others. I have tried to, include as much as, I could...with the time allotted.
I myself, wish I knew the names of the alleged, Chief Spotted Tail's 2 sister's.
Page 22 in The Killing of Chief Crazy Horse: Three Eyewitness Views (introduction by Robert A. Clark) says Spotted Tail was Crazy Horse's uncle, and he used Hyde as a reference in his footnotes, but it appears that is wrong. How reliable is the other information in George E. Hyde's Spotted Tail's Folk? Some of Hyde's work was published decades after he began researching in 1904, so the publication dates can be misleading. Here's a link to Hyde's Spotted Tail's Folk (page 15, that says Crazy Horse "was the son of Spotted Tail's sister") If it doesn't show up, hit reload and that works for me. books.google.com/books?id=ZbkNJpW-BAwC&pg=PA15&lpg=PA15#v=onepage&q&f=true ..... Off topic: Page 98 in The Killing of Chief Crazy Horse: Three Eyewitness Views (He Dog, William Garnett, Valentine McGillycutty) shows Billy Garnett misidentified the relationship between Woman Dress and Louis Shangreau. He said Woman Dress was Shangreau's uncle. I borrowed a 1st edition copy of that book and scanned every page last night. I was surprised to see there were only 300 first edition copies published.