[Tagged as NSFW for discussion of sexual topics.] Alright, so this situation is super messy and I lowkey need some advice because everything is turning upside-down. My parents have been married for 22 years; when I was 9 years old, they introduced a third partner, making them into a triad. I am now 20 years old. I never got along with my "step mother" and two this day our relationship is nothing more than us considering eachother immediate family and occasionally sharing small talk. But thats a story for another time and place. (To be clear I am not shaming my parents for being poly and I do not have issues with that; I have a rough relationship with my "step mother" because she was neglectfil and verbally abusive, but my bio parents have still kept her around for well over 11 years at this point.) Since I was a child, my parents have always tried to set the example for the perfect couple; never considering divorce, always talking through their problems, being the "perfect parents" to their three kids, and being madly in love. However, the older I've gotten, the more I've come to realize this is just the display that they put on and their relationship is much worse off than they'd ever admit to us. While I have a Rocky relationship with my parents due to childhood neglect and heavy trauma they "don't believe I really have", I still care about them deep down in my heart and as a kid I looked up to my dad more than anything I wanted to be just like him when I grew up. So did my little sister, who seemed to spend her life so far idolizing my dad.
Now, here's where the crazy stuff starts. As I said before, it's not the poly I am shaming my parents for, as you'll learn. To make a long story short, my parents are also swingers, despite them being in their late 40's. They frequently go out to party, do karaoke, drink at people's houses, and of course...sleep with a lot of people, and I mean a lot. I didn't find out about this until I was 19 years old, but according to my mom they have been swingers for at least a few years now. My mother admitted to me in confidence what my parents got up to, even oversharing about sexual details like her kinks and the things she does at the swinger parties. Now this is something I do not appreciate at all as it makes me very uncomfortable because I am literally her son; however, in the past I've tried to brush it off because I wanted to keep my relationship with my mother. My mom has told me many many things no child should ever know about their parents; but I just recently found out something that makes me ten times more disgusted than anything she's ever told me.
Yesterday my little sister who is 13 called me on my lunch break while I was at work; of course I answered. She dropped the absolute bomb on me that she found out a nasty secret my parents had hidden from us. How? Turns out my mom gave her an old MacBook that still had all of my mom's old iPhone messages in there, and unfortunately my little sister is a major snoop and looked through many of her texts. On the call my sister told me she had found out my dad had an affair on my mom that lasted a WHOLE YEAR and got the woman pregnant!! Allegedly she got an abortion and my mom only found out about the affair from the mistress. My mother still chose to stay with my father. Now I don't know how long or how recent ago this was as the messages don't specify time or date, but I know my mom said it took her a year and a half to trust my father again. Why? Why would she stay?money apparently...is the common answer I've been finding everytime my mom is about to leave my dad but doesn't. She's even had mental breakdowns and even gone to the mental hospital several times due to my other parents and their lack of maturity [per say]. My mother is still not really a good person but I don't believe anyone deserves that treatment. Either way, circling back; I'm beyond devastated to find out about this and now I don't even know how to look at my father or my moms. They kept this from me from only God knows how long and I only found out about this because of my 13 year old sister snooping. This is beyond fucked up to me and my sister is no doubt never going to look at my father the same again. I live independently from my parents with my girlfriend and have offered my home as a safe space for my little sister as she needs one and even offered to take her to school on the days she spends the night, but obviously I can't force her to confide in me or leave home, more could I take 100% care of her if my parents were to become non-existent; I have a full time job and too poor of mental health to take full responsibility of her but I want to be there for her unconditionally. But I'm scared for my sister; she's only 13 and has a history of self harm and [a] mental hospital visit and is suspected of BPD but is too young to diagnose. I don't doubt that she's not going to be okay and I'm terrified. If anyone who has been in similar situations could offer me some advice. I'm afraid and I've already gone low contact with my parents before this but I don't know what to do after this.
Please feel free to ask questions as I have only added bare minimum details needed for full context.