it's either that they had an ambiguous relationship when they were kids and he tried to rekindle it (she liked it too it seems), or some blackmagik fuckery with the boy giving her a heart (if that was the case, the parents n relatives would've had more expressions in their faces) or breast implantation (that too would've been too controversial to jus be like, 'hey my cousin I don't remember has got no tiddies while I do now, so let's go the wincest route' the people are going on about below me. Either way, I am not gonna go too deep into it since there should've been more context to have a deeper meaning...