Maybe I need to get to the later games, but I very much read that as a 'like brother and sister's situation.
Plus in the first game they're at just the right ages (17 and 24) to make Maya/Phoenix creepy. Maybe I'll see it in the post-time skip games.
I might just be overthinking it, but I'd definitely be more comfortable if the robe looked better secured.The robes have to be loose to accommodate all body types, and a camisole would end up ripping. It's fine... and also we see something far far worse later.
Mia can bare her breasts if she likes, however she doesn't have the right to do the same to Maya's. Plus I am very definitely overthinking this.