To be fair to K, she likely knew that other kinky people were there and may not have had any inkling that sister and BIL were the exceptions. I think she showed poor judgement in initiating contact, for several reasons, and hopefully she's learned from this, but it was hubs who KNEW this was not ok and yet went along with it, so I can see how she'd assume from that that nothing was amiss. He's the one who seriously bounded across a line here.
I've known a couple of people who were into getting away with sex in public and I just don't know how to resolve the fact that I find them to be caring, respectful folks and yet they're willing to potentially put other people in extremely awkward positions because they find it hot. What the heck? This is one kink I just can't get behind.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.