If I really am getting the read that she is going to try it without my permission or behind my back, then I see myself as having to choose between three bad decisions.
Bad decision ONE - She takes it on her own, maybe with her sister participating maybe not, and tries to hide it from me.
Bad decision TWO-She talks her sister into letting her give HER an enema, without my blessing or presence.
Bad Decision THREE-I give her a medically unnecessary but carefully managed enema with everything explained twice, once just the two of us sitting by ourselves, Again while I'm giving it to her, offering to let her do her own insertion.
While non of those things would be an ideal situation, parenting is all about non-ideal situations and adapting.
I consider Decision 3 to be the least bad alternative.
Soooo, as my father told me when I was just about her age, and I asked him what he defined as being "grown up" he said, "Being able to be left choosing among bad decisions, making the less bad and walking away knowing you did the best you could, even if that was not very good