But you aren't changing anyone's life. That third life wouldn't even exist if it wasn't for those two specific people having it. If I was right now to choose between being born with a birth defect vs not being born at all I'd take the birth defect. After all, the inbred person can later in life choose to commit suicide if they want to. I'd personally be okay with killing off the inbred children that literally have no agency and are braindead, but if they just have some birth defect that makes their life a bit harder I don't see why we should be worried about that when the alternative is them not being born at all.
Plus incest doesn't have to result in a child. Would you allow two brothers to have sex, or two sisters, or a brother and a sister using protection?