This is a story I read on reddit a long time ago and cant find it anymore. A brother and sister home alone for a few days while their mom is out of town, the brother convinces the sister to massage his cock and they end up having sex while the mom is out of town. Their Aunt who is their moms twin comes to check on them and realizes what has been going on, she quickly joins as well. The three of them go shopping one day and without knowing he runs into his mom and has sex with her in the changing room thinking its his aunt.
Source: reddit.com/r/Erotica/comments/mrvrp8/help_finding_a_story