I've found very few references to female Sith. I know there were some powerful female Sith, but there aren't many of them.
Is there a reason there are so few female Sith?
I looked through the Wookiepedia, including the main article on Sith, and there was one female Sith from the time of The Brotherhood of Sith and two (or three) other than that, and that was it. While there aren't many female Jedi, there seem to be fewer female Sith.
This is a case where I'd accept an answer like @Sydenam's or @Aye's comments, as opposed to a purely in-universe answer. Or maybe it's that there are fewer female Force-sensitives than male ones. But whether it's due to sexism, or that male authors write better male characters, or that there's something in-universe that causes it, there's still very few female Sith, so it'd be interesting to know if there's a reason for that.