[@Gunther] Thanks homie! Glad to have you here. I have always known you to play fleshed-out martial characters, and that is currently exactly what we need. Short answer to the Sisters question is "Unfortunately no, Brienne was kind of an exception to the rule" but I don't wanna sound dismissive, so I wrote up two consolidated reasons why I think you might have trouble with the idea in practice.

1. Sellsword companies are an Essosi construct because they lack the holy order of knighthood altogether, not following the Seven. There are mercenaries within Westeros, but they carry a negative social stigma and do not organize into companies, as they are essentially illegal hitmen. The Brotherhood without Banners was more like a rebel group that existed because of the political climate, chaotic warring taking the attention of high lords, and the increasing popularity of The Lord of Light because of Beric and Thoros (And Melisandre, elsewhere). A sellsword offshoot of the Brotherhood could likely thrive in Essos, which is reeling from the chaos of Daenerys, but not Westeros, which has been neatly brought to order by two monarchs within a few decades.

2. Brienne, aside from being all about honor and knightliness rather than financial gain presented by selling her sword, was a very big woman. By Martin's own words, she was as tall as the confirmed 6'6 Bobby Baratheon. I don't think she'd associate herself with killing for hire, even if she was keeping peasant girls safe and starting a female martial institution. I don't think Brienne cared as much about "Women can fight just as well as men" so much as "I, Brienne, can fight as well as men." I also don't think a female group of women could safely exist in such a misogynistic society -- this would be a group whose very existence insults Westerosi conventions, whose members would likely live under the constant fear of being captured, raped, and killed at all times for being female warriors. Not exactly good for recruitment.