Actually, the super-messed-up genetic disasters we think of come from inbreeding practiced over many generations. Your 'normal' sibling couple, were there such a thing, has a very small increase in likelihood of birth defects. (The things you learn from the Len/Rin Kagamine shipping community... though that really does check out. Easily google-able, if anyone cares.)
Of course, that doesn't do much about "feeling wrong seeing something like it." But hey, live and let live.