“Sisters of battle” probably don’t fit the “low medieval/dark fantasy” thing.
But you could maybe have brothers of battle that were women in disguise. It has happened in every era of humanity. Tricky to do well, but it could be a special background, a chest covered in tight bandages/undershirt (opposed to naked for standard bro) and some related events. No need for special heads, standards one will work well. (and the player can alway take off the fake beard at the barber if he want)
I say “tricky” because no matter the way you do that, someplayer will be offensed. Everyone is offensed, theses days…
It’s maybe too much work for the addition of just a background and some events. (even one “undershirt” chest is a lot of images I bet, if you want to have all injuries)
But if done well, it could add a thing. Women disguised in male soldiers is a common thing in books ^^ The Black company has (at least) one I remember. The Monstrous regiment of Pratchett (rest in peace) has too. And you will find examples in real stories.