I read this a long time ago on reddit and cant remember where. 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. After the mom returns the brother mistakes her for his aunt and has sex with her, this causes the mom to join in their sex adventures as well.
Source: reddit.com/r/eroticliterature/comments/mrvq0u/help_finding_a_story
Jesus. Count me amongst its hunters.