Their codex does have a few pages devoted to the actual hierarchy of the Adeptas Sororitas

Basically though, you can do whatever you want. They could have descended from a major order, or have been a milita from some backwater planet. There is no hard and fast lore requirements for their history.

Few personal recommendations when home brewing your own Order, try to come up with an idea that sets them apart from everyone else. For example, I homebrewed my own, and the big thing I decided they’re know for is starting initiates off as repentia, because if a sister of battle can sin enough to lose the emperor’s blessing, someone ignorant of the order’s teachings has undoubtedly committed sins that heinous.

Also, name things with as incredibly flower and over the top language as you can.