Met "Jill" about 4 years ago. She was a total wreck at the time.
"Kate" is her daughter and my soon to be sister in law. We do not get along at all.

Kate and brother came to live with me about 6 months after they got together. Jill was not allowed to come over, daughter there or not, she was not allowed.

Wound up kicking Kate out after about a year. Brother, Kate and Jill started living together. Within 2 months they had no place to stay. Told brother he could stay, the girls could not although I did let them store their stuff. Leaving Kate and Jill homeless basically. The girls would come over at times to visit and get some shelter (it was snowing outside at the time) during the day. They all got a place rather quickly.

After this we would get together and hang out at times BBQ'ing, Thanksgiving, Christmas etc. Jill got better, much better during this time as well.

Year and half ago, me, my brother, and Kate was back in the same house again. That lasted about a year, and told Kate she had to go. She moved in with her mother. Brother got sent to prison for 6 months a few months after. Soon after Jill and I started hanging out and partying at times together while Kate is at work. Kate does not like this at all and gets pissed every time she finds out.

Jill has always wanted to have sex with me and has always been vocal about it. The other day I laid out the rules. Neglected to tell her, Kate could not know. Due to time, logistics etc... we could not do anything until early this morning. Did not expect her to be coming over, had used up my condoms elsewhere so could not do anything and just made out. Kate has also been trying to get Jill into a relationship with someone else. Jill does not like this other guy in a sexual way. She tells Kate we made out and what the deal was. Kate flips out, she does not like us alone talking, much less anything else. Jill tells Kate that Jill and I are adults and we will do what we want and we are hooking up tonight while Kate is at work.

Kate calls me after I get off work (and I am supposed to be going to meet Jill) giving me her opinion, don't do it etc... I told Kate it wasnt to piss her off or anything, she knows I do what I want, and dont care what other people think about it. Kate then tells me Jill is going to stay in bed tonight, and Jill does not answer the door when I get there (her phone is broke).

This crap is one reason I never went there with Jill. Jill is cool with it, as long as we are having sex that is what she wants. Kate will probably do whatever she can to interfere.

I have zero doubt Jill is going to show up at my house around 3-4 in the morning. Should I say fark this particular fwb situation rather than farking her? Kate will be my sister in law in less than a year so I am stuck with both of them being around at times.

Ironically at one time way back when (shortly before Jill was not even allowed at my house), Kate was trying to get me into a relationship with Jill lol.