It seems odd, doesn't it? When most people say "incest," they're thinking of parents coercing their children into sex, something which already clearly falls under the heading "rape." Tossing in "or incest" doesn't make sense unless you specifically want to exempt consensual familial sex as well, presumably between siblings.
I'm not sure why a pro-lifer would want to do this. It couldn't simply be because he disapproves of such unions; otherwise we'd hear people attacking abortion "except in cases of rape, incest, or adultery." And it can't be because the baby's more likely to be born deformed; that would fly in the face of the entire respect-for-life argument.
So why "or incest"? Talk about your perverse incentives! Under such a law, the government would basically be saying, We won't allow you to use abortion as a means of birth control. Unless, of course, you do it with your brother.