MIL guilt tripped her “golden child” over missing nephews birthday party for big job opportunity, and somehow I’m the villain! Help please!

Context:
Fiancée (34M) and I (33F) have been dating 5 years, known each other for 15. Dated for 2 years when we were teens, had a very fun and nice relationship. Broke up due to distance. Didn’t speak for 8 years. Met up for dinner, been dating ever since, now engaged.

My MIL(54F) is very controlling, very obsessed with her son (my fiance) they call him the “golden child” and his sister slightly resents him and their mother due to the favoritism. MIL/FIL marriage is very, very rocky…only stayed together for financial and social expectations. FIL has cheated at least twice that we know of. MIL has suspected bipolar and anger issues, not diagnosed. MIL has been violent towards SIL, confirmed. MIL/FIL are violent towards each other, so I’m told. This family pretends they are the most perfect cookie cutter white picketed fence family on Facebook. Everything is about optics and for performance.
My fiancee is the black sheep, the exact opposite of all of them, thank God. He is absolutely marvelous and we have an incredible relationship with nearly zero conflict and our own business that does very well. We are very, very happy and have the best life. I can’t express how grateful I am to be with him.

I’m looking for advice on how to deal with my MIL/SIL.
I will give an example of what happened this week so you can get the vibe of these people and how they operate.

We told his SIL(30F) we would go to her 3 year old’s birthday party next week. We then had an amazing opportunity come up for our business, coincidentally on the same weekend as the birthday party. We both knew we couldn’t pass it up, so we told the family we won’t be able to come to the party but we will make a special trip down to their house (about 5 hours away) in 2 weeks to make up for it.

His mom sent him a ChatGPT novel text, guilt tripping him and talking about how “mommy just misses her baby boy so much, it breaks my heart that we haven’t seen you for a few months. You make time for all your vacations, but no time to come see your mommy and dad who love you more than life itself? Family is the most important thing, and we’re heartbroken that we aren’t a priority to you anymore”
Fiancée politely told her that he would prefer she uses her own words instead of ChatGPT. His mom dropped her usual fake, phony saccharine act and showed her true colors. Name called and got defensive and rude and then started ignoring him.

Then SIL called, he told her she was on speaker and him and I were on a walk in our neighborhood. She proceeded to guilt trip him and harass him and accuse him of “not caring about her kids and not doing enough for her kids”. Note: 6 weeks ago, we hosted his sister and her daughter in one of our rental units, and paid for them to go to universal studios and paid for all their food. Spent probably $1,500 hosting their weekend trip. He reminded of her of that, she accused him of being cheap and holding that over her head. SIL then said that because we run our own business we should be able to take more time off to come visit them. My husband was silent and just looked at me defeated because he had repeatedly politely responded to her over and over and she just kept berating and wouldn’t listen. I calmly said in the background “We have a lot of responsibility with our business, and we’re doing the best we can.” And APPARENTLY, according to MIL and SIL, I’m the villain for “inserting” myself into the “family conflict”…
(I previously had had no conflict with either of them up until this point as I choose to stay out of it because I do not like their inauthenticity and vibe)

I’m beside myself. They spend the majority of the time being saccharine sweet, fake and performative as if they’re the best, sweetest family on earth…but a few times a year they put down their masks and reveal that they are rude, petty, selfish, demanding and miserable underneath it all.

Any advice on how to deal with this? My fiancée has grown leaps and bounds in the last five years and is able to stand up for himself in a polite but firm way, I am so proud that he doesn’t let them walk all over him anymore for the sake of “they’re family and he loves them” BUT they are not used to him pushing back and setting boundaries… and when he does it causes days and days worth of more drama and guilt tripping from them, which makes him miserable because he is a very non confrontational, sweet and calm person.

Any advice on how to navigate would be greatly appreciated! I’m open to ANY advice or tips. Please ask if more context is needed.
Thank you!!