Yeah but prostitution is also illegal and morally wrong an they still used it in the game so they could probably do the same with incest
Well yes Prostitution is legally and morally wrong, yet it is hardly comparable to incest in how morally wrong it is.
Prositution can be shown in a game if its handled with care, in FFVIIs case they used Humour.
Incest however is simply a Taboo subject - and will never be used in any game.
so Again Aerith and Sephiroth could ever only be siblings OR lovers - not both.