[m4a] My MOST Embarrassing Fantasy

Hello there, I have dreamt of this moment for a while now. It's something that I think about when I see her at family dinners and especially at family vacation. It is also something that will never be spoken of, except for here. Just typing this out makes me a little nervous/jittery. I feel terrible that this is a fantasy of mine. It started before my wife and I got married. I would go over to their house, and she would walk around in a crop top without a bra and Spanx because she was so comfortable around me. It wasn't always like this. For the first several months, it was an oversized t-shirt, sports bra, and shorts/sweats. Then it changed. I couldn't help myself but to sneak a few glances her way. I thought to myself, 'I cannot be doing this. I cannot be fantasizing about my wife's younger sister.' 'But she's just my type.' Short, small, and somehow a rather large chest for her size. Nothing like her sister who is taller, not as small, but smaller chest. I can't think of this. It is so taboo. So bad. Sometimes I imagine what it would be like if she would walk in on my wife and I having sex. Would she turn and run in horror? Would she stand and watch for a couple seconds? Would she even ask to join? Or would she wait to get me alone once my wife leaves for work the next day...? If you want to chat about embarrassing fantasies or if you are interested in RP, pm or chat me!