Before I cum while keeping my body stiff like that, I always make sure her legs are up in front of my arms and her wrists are held down on her sides with my hands. That way she's completely helpless, pinned and ready for me to put a huge load inside her. Once I get every drop inside her, I move forward even more to get her ass raised in the air even higher and hold her like that for at least 2 to 3 mins and there isn't anything she can do while my cock stays plugged inside her keeping it sealed.
It's too late to worry about the consequences of a vaginal insemination when the sex is ince*st or close to it. Family sex is always risky whether it is a blo*od relative or just a step relation. The entire concept of family sex is considered taboo in this society even though the act is filled with extreme pleasure. At least that is what my wife tells m about her sex with a sibling.