Despite the name, nobody in the Le Fay Sisterhood are sisters. Some of them are, but the majority in the group are not. It's also not exclusively for feminine folk, the name was just kept like that because it sounds cool.
The Le Fay Sisterhood was founded by the legendary chimera Morgan Le Fay (not to be confused with the Morgan Le Fay of Arthurian legend) for anyone who practices magic. If you're a member and you get injured, discriminated against, or otherwise treated awfully, then the Sisterhood will cover you! They also have facilities and a ton of branches in various places, such as...
Myosotis Magician Association (Formerly the Le Fay Sisterhood of Myosotis)
Founded by Stace, grandchild of the greater Sisterhood's founder, it does exactly what you'd expect for a branch of the Le Fay Sisterhood, save for one thing:
Necromancy.
Stace is a necromancer, and uses the Myosotis branch as a front to help advertise her services.
As it turned out, Morgan and Stace's mother didn't appreciate the Le Fay name being associated with necromancy, and forced her to stop using the Le Fay Sisterhood name for her branch.
Nowadays it's known as the Myosotis Magician Association, and does the same sort of stuff it did before it changed it's name, but without the necromancy as Stace quit that stuff after it started physically affecting her.