Struggling with My Mother's New Marriage and My Sister's Reaction

This is a very long rant that kinda leads nowhere, but any help is encouraging: I’m really struggling here. I feel like I am being forced to pick sides. My mother recently remarried and my older sister hasn’t taken it well at all. Let’s just say our mom has almost always put men first. Our lives have been flipped upside down multiple times, being forced to abandon our homes and many belongings and friends in order to move, either away from an ex, or toward a new guy. Our mom has gone through years of emotional and some physical abuse, causing her to be pretty bad at picking reliable and trustworthy men. She also grew up in a household that enforced male validation, “you’ll never be anything/life will be too hard without a husband to support you”, “if your husband is upset with or attacks you, you should reevaluate what YOU are doing to make him feel better so he’ll stop”. Pretty twisted I know. Obviously, now at face value she understands that is completely wrong, but she retains some of the habits from this lifestyle. And now that she has a husband she feels she can trust, she puts his needs before her children. I’m not going to lie, it did and does feel like we were mostly tossed aside to an emotional extent. But we can’t really get through to her that she has done anything wrong. We were all still recovering from the last man, my youngest brothers father, who did a great job at making us all feel worthless. It feels like she rushed us into a relationship we kids weren’t ready for. My little brother and I mostly dissociated throughout the hole thing, and still hardly have a relationship with our new stepdad a year after moving into his place. My older sister on the other hand has been very assertive from the beginning, clearly weary based on past failed relationships. I’d say I kinda like him. It took me a while to adjust, but he’s definitely the best so far and nobody is perfect. Anyway, this is where I start to feel torn. While she puts up with him and is overall friendly, she cannot stand the intimacy between them. She is very sex adversed. I guess I should mention she is autistic, and while she is almost a fully developed 25yo adult, her emotional and sexual maturity is still akin to a child sometimes. She experiences full on mental breakdowns, and will attack my mom with very harsh words when they are overheard being intimate in the bedroom. Yea, it’s effin gross, nobody wants to hear their parents like that. But my brother usually just turns up his video game volume and I turn up my music. On one hand, I know they deserve to be intimate as a married couple who is still clearly in their honeymoon phase, but on the other, they both completely dismiss and invalidate her feelings. It seriously bothers her, and instead of trying to help and hold off on the sex for like two seconds while they get her some therapy, they just sort of laugh it off to her face. It kinda disturbs me how little they care about her emotional well-being when it means they have to sacrifice something. Instead of evaluating why my sister is lashing out, or when any of us lash out for anything, she forms herself into the victim, and reminds us how much we owe her for keeping us alive and with nice things. I know intimacy is important for the health of a relationship, but my poor sister is losing her mind. And I hate being caught in the middle of it. I know this is kind of a hard one for some people, at least from what I’ve seen online. Any tips on how to reach a stubborn parent who never feels they’ve done any wrong and lacks most accountability when it comes to acknowledging the years of emotional incest she’s inflicted on her kids? Also maybe in a way that makes her feel like she thought of the idea, since when I try to approach a situation with her using my upmost maturity and real professional advice, she accuses me of “playing counselor”. Also, side note, I know my mom can clearly sound like a bad person from this info, but this is her at her worst. She has many times been exactly the mother we need and shown that she is very loving person and cares about her kids immensely. But she has her issues. Seriously. Which is probably why I’m struggling to see two people I care about tearing at each others throats.