Withdrawing a cock and then putting it back in is stupid if you are trrying to avoid a fertilzation. It only takes one drop to do the dirty deed. Why not put your stepdsughter on the pill and cum in her pussy? She will enjoy it more and so will you. If you are having family sex that is borderline ince*st, you may as well "go all the way" and pop your load in your stepdaughter's pussy.