Even though both were transgender in original Final Fight, in later years only Poison's gender has been a matter of contention, and Capcom at one point said that Roxy "isn't into the whole crossdressing thing (unlike Poison)", which means, given they look almost identical, that Roxy's a girl. But this is a nice illustration, I think we can let it slide.
(And besides, they're kids. They shouldn't look that different at this point.)