There is a book just published called "Metis Families: A Genealogical Compendium" by Gail Morin, published by Quintin Publications, Pawtucket, Rhode Island. 2001(ISBN: 1-58211-412-9). It contains great long family lists of Metis families which might be very helpful.
I found it in the reference section of the library.
It doesn't list her mother's name (lists her as "Swampy Creek Indian married to Louis Riel according to the custom of the country before 1840") but here is what is says about Marguerite Riel: "born circa 1840; married Jean Baptiste Zace, son of Louis Gonzague Isaac Zace and Angelique Pariesien, before 1861. died 7 May 1874 St. Norbert (MB), buried 9 May 1874 St. Norbert." sources James Riel, IBMS This listing was from volume 5.