Can´t remember all the details but a mom and son are transportes (I think through a mirror) to an alternate reality / parallel dimension where there are no tabus about sex / incest / clothing and end up having sex with all their family. Also, that reality’s mom and son are transported to this one and are having trouble adjusting.
Source: reddit.com/r/Erotica/comments/lx8396/looking_for_a_story_about_an_alternate_reality
I like alternate reality stories, so if anyone finds it, please let me know.