Originally Posted by
Digifiend
Have they ever been exposed to Terrigen? It would have no effect on baseline humans and mutates. Consider the new Inhumans like Kamala. Her parents don't have powers, but one of them must have the Inhuman gene - they just didn't get any powers because they haven't been exposed to Terrigen. You can be an Inhuman and not know it.
At one point, Luna Maximoff not having powers would've proved that anyone who has a mutant child (which would include Reed and Sue) can't be an Inhuman - the Inhuman and X genes cancel each other out, making offspring normal baseline humans (a survival mechanism, since we know Terrigen is toxic to mutants). However, then Luna's father Quicksilver was retconned to not be a mutant.