Eh, they've had separated children meet up years later and fall in love because of a "Connection" and almost always when they found out even after they married they divorced or stopping doing or wanting to do "that".
I still see it as all kinds of wrong for a sister, even half sister, to lust after her brother even after learning that fact.
I honestly hope that sibling thing isn't really a real thing but as it's Muv-Luv I wouldn't put incest out of the question for Yui at least.
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