I fucked my partner in some one else’s panties [FM]

So a little background: I have a fairly high sex drive, to the point that my partner and I allow me to see other people. Covid has made this a bit harder. But I finally found a new sex friend a few days ago. I’ve been dying to meet with her as she’s been doing lots of teasing. We finally settled on a meet up day and then…. company was coming.

My sister in law came over. For one. Whole. Week. She broke up with her boyfriend. She lives nearby so we offered her a couch to sleep on while she got her affairs in order. My partner and her are closethough, so they were happy to spend time together. She’s cute though. Similar body type to me, brunette. Slightly bigger boobs….maybe.

Anyway, over the course of the week she stayed, I had done her laundry a couple times. Evidently, a couple pairs of her panties got mixed up with mine, and I ended up accidentally adopting them after she’d left. I hadn’t even noticed until I was getting dressed for work a couple days ago. I figured “why not?” and put them on. I was in a bit of a hurry, so I didn’t think much of it. But after I settled down from my rush, I realized just how fun and sexy it was.