There's always a power imbalance between parents and children, no matter what their ages, such that a relationship between a parent and a kid can never be consensual. As to siblings, siblings who grow up together almost always learn early an evolutionarily ingrained bias against sexual attraction to one another (apart from innocent childhood experimentation that they don't understand, maybe. And obviously this doesn't include sibling abusers, who get off on the transgression and on having power over someone.) But in the extremely rare cases where siblings transgress this, whatever. I can honestly say that I don't care. It grosses me out, I'll be honest. But I don't care. If it became widespread we'd have a reproductive issue. But it never will become widespread, due to the ingrained reflex against it. So... whatever.
Of course this person probably doesn't admit that evolution exists, in which case I'm sure Christianity and its tangled, contradictory laws in Leviticus and so on are the only things keeping siblings everywhere from fucking like bunny rabbits. ::eyeroll::