I get where you're going but it's too complicated for my lack-of-sleep-addled-brain.
I've seen it time and time again on this forum, and have first hand experience in real life. People are far, far more upset at the idea that a married person would be playing with encouragement and permission than they are with cheating. It's utterly illogical, but it is what it is.
Petra, it's very sad about your sister. I'm curious, and I hope you don't mind my asking, does your father know about her escapades and what is his acceptance of her?