Diana,
My sister and I also had contests. As most readers know, we got enemas from our grandma from earliest memories. These were administered in her bathroom, kneeling together on her tub, naked. She had one of the few houses that had a guest bathroom back then, and after we satisfied Grandma that we had held our enemas long enough, Sis got that toilet, & I ran for the guest bath. After she became klismaphilic, about 14, she suggested that we could do each other when Grandma was out at work. I wasn't really into enemas yet, but didn't mind getting them, and thought giving them to Sis would be fun. Before long, we started playing enema games: most volume, strongest soap solution, longest hold, most poop in the toilet. By 16, I was losing every time, because Sis was rubbing her clit during the contest, and only going after she orgasmed. The first time, it scared me half to death: she got rigid, shook all over, and her eyes rolled back in her head. Then she let out a muffled scream and collapsed in a heap. I tried to help her, but she laughed and said, "OMG, that was the best feelings ever! I want to do it again after I shit!" It wasn't long before we evolved into mutual masturbation with our enemas. Despite being naked and masturbating each other, we never actually had intercourse back then. Now, we are both in our sixties, retired, and divorced/widowed, and we see no reason to stop short of full engagement. She still screams, and if we are in the back yard, expels forcibly as she orgasms. Sometimes she jokes, "Think a 66 yr old could get pregnant?" (That one can't...she had everything removed 10 years ago.)