We just don't know. She/he had an exposed nipple in the artbooks and people drew conclusions. I guess they're supposed to be sisters and people are just being paranoid, but better safe than sorry? Maybe?
There's also the matter that the QB anime does not enjoy showing the nipples of the lolis (Ymir mostly Erin and Huit will likely follow, Nowa isn't a loli, she's around Elina's age).
Then again, I haven't checked out the Rebellion anime so I dunno if that's changed all of a sudden.
Which is basically the same arguments from when people saw her from the books, didn't help that the previews for her/him kept saying there was a 'secret' about the char.
It may be that in the end, hobby japan has just decided to retcon her being a boy but until we have official word, there's just not enough concrete evidence to say she's a girl with 100% certitiude.