[[ Well, using Sasuke kind of negates the question. There’s no sister for him to reproduce to restore the clan if he’s the last one left, is there? And if Itachi is this sister, it still doesn’t work, because Sasuke will want to kill Itachi. 

And I have always hated the idea of a female relative OC introduced purely for Sasuke to have sex with. And I’m not saying “ew brother/sister sex” because I already ship incest type ships, so that’s not the problem. It’s the canon dynamic being introduced by forcing a relative who’s only purpose is to hook up with a canon character. 

Unfortunately, this kind of stuff doesn’t work in a non-massacre scenario, either. The Uchiha isn’t as strict or crazy about their “breeding,” or the importance of their kekke genkai, so Itachi and Sasuke would be expected to marry women, and Uchiha women at that, but nothing as close as a biological sister. 

Perhaps in some other, very specific setting, where there are very few members left and even less females at an age they can reproduce. Something as serious as that would be applicable then. ]]