FYI, I just found this website of "Classic and rare R&B/Soul Sisters ('50s to '70s)"
http://rateyourmusic.com/list/nosialmenara/classic_and_rare_randb_soul_sisters__50s_to_70s_

This list was compiled by nosialmenara, who also has a blog on this subject at http://supersoulsisters.blogspot.com/