Genetic attraction is a theory that say that if two reletives meet, who don't know they're related, they have a greater chance of being attracted to each other. In short, if you didn't know sis was your sis and that she was all gross and stuff... you'd probably think she was hot. <I'm not making this stuff up, honest>
Therefore, isn't it possible that whatever genetic differences that the heroes have in common are acting in the same manner. We just don't notice them because we are just normal humans.
Sorry, I know this is speculation, but to be fair, I didn't start it.

Good news, though. Lot's of super hook-ups and super/super babies!