I thhought this was always sort of a 50/50 thing. They may inherit their parents poweres (Polaris) or have totally unrelated powers (Qucksilver/Scarlet witch). But they did always touch on oddities between relatives and powers, such as Cyclopse and Havoc, their powers don't affect eachother. I think it could go either way cuz that's how genetics work. It's just a crapshoot as to which genes are dominate, like two blonds giving birth to a brunette, sure they're blond but that doesn't mean there's not a brunette gene floating around inside them somewhere.