I had been living with Cynthia, an attractive, 46 year old woman, for about a year when her 42 year old step-sister, Heather, moved in with us. Heather was a real good looker too, at 5'4" 118 pounds with really firm 32B breasts. Heather moved in after breaking up with an abusive boyfriend she had been living with. She and Cynthia were pretty close for step-sisters. Cynthia's father had married Heather's mom when Cynthia was 20 and Heather was 16.
The first time I saw Heather was at party for Cynthia's dad. It was a pool party, and the stepsisters were wearing very skimpy bikinis. Even at 41 and 45, both women were really hot in that attire. In fact there were some younger women at the party that got rather upset their husbands were eyeing the two. Cynthia noticed me casting more than an occasional glance at Heather and playfully teased me about it. In fact the "teasing" became a regular part of our sex life after that. Cynthia would taunt me saying things like, "bet you'd like to see Heather's tits wouldn't you?" She even occasionally made references to what it might be like to have Heather in bed with us, well that would usually result in me popping my rocks right there!
Naturally when Heather moved in with us, I started having visions of threesomes dancing in my head. Cynthia put a screeching halt to that notion when she pointedly reminded me THAT (the earlier teasing) was pure fantasy and for me not to even think about it. She made it clear to me her stepsister was strictly off limits. I believe she also had a similar discussion with Heather, telling her I was off limits. The only thing was we (Heather and I) were attracted to each other. I got a sense of it one night at dinner, catching more than one or two glances from Heather. Cynthia caught it too and responded with an icy look at her stepsister that for the moment squelched the visual flirting.
Up to that point, whenever Heather went into or came out of the shower she would be completely wrapped in a huge bathrobe. Then she began appearing wrapped in only a towel, and the towels kept getting smaller. There were other things she never missed a chance to walk past me in a manner which would enable her to brush against me. Cynthia picked up on some of these things as well, filling in the blanks with her imagination. I came home one afternoon to find the two of them in a screaming match. Apparently the argument started over some trivial thing, but fueled by the growing rivalry, the minor disagreement quickly escalated into a loud verbal argument. They were facing each other, almost nose to nose. I think had I not walked in at that moment, the argument would have escalated further into a physical altercation. When I came in they both cooled down and things seemed to settle down between them, it was almost as if the shouting match was a safety valve that let them blow off steam.