ILs, "sacrificial lamb" dynamics and dealing with anger

My fiancée comes from a highly dysfunctional family - alcoholism, teen pregnancy and domestic violence in the extended family (they all lived on the same property when she was growing up so these things were part of her daily life), FIL walked out on MIL, ghosted his daughters for years (including not paying any child support), was never really there for them, yet expects obedience and attention (without having even met me, he started a smear campaign against me because I didn't ask him for his daughter's hand in marriage, he complains none of his children contact him but won't deign contact them, etc). MIL was a harsh disciplinarian when my fiancée (older child) was a child, but also failed to protect her (my fiancée has been diagnosed with PTSD from childhood trauma by a psychiatrist, and is in therapy), and at some point seems to have given up (and was a much mellower parent to my fiancée's younger sister) - MIL became a doormat, and an enabler of my fiancée being in a family servant/sacrifical lamb role. She wasn't a classic "scapegoat" in that her parents didn't relentlessly humiliate or blame her, but her needs and wants were consistenly put last, which severely affected her education. Also, when it came to bearing burdens (not least financial) she was treated as an adult from a young age (which didn't happen to her sister or cousins) but was never included in any decision-making and didn't have freedoms matching her responsibilities - so she was both parentified and infantilized. She's in many ways remarkably level-headed and well-adjusted given her past, but has trouble for example prioritizing her health now that we're expecting, is hypersensitive to feeling excluded and sometimes has trouble communicating her feelings (in my job I deal daily with precisely defined technical terms, a bias that has contributed to miscommunication as she can struggle to find the right words). We met in her hometown, and when I took a job where we live now she moved in with me. She struggles to make new friends or even acquaintances - I'm not very social myself but I've made friends at work.
While to some extent my fiancée agrees she was "thrown under the bus" she also views it as a story of her own decisions - but these decisions didn't happen in a vacuum and it was very convenient for others that, for example, for years she was the main breadwinner in her immediate family and still did housework and didn't even have a room of her own.
What prompted me to join this community and post was something she told me about one time she confronted an alcoholic, wife-beating uncle-by-marriage - he's a coward and backed down when confronted. Afterwards MIL chastised her - she's positive it wasn't due to fear for her physical safety but rather out of a don't rock the boat/don't talk back to your elders thing. Hearing this was a kind of straw that broke the camel's back moment for me.
So I'm here partly here looking for insights, etc as to how to help my fiancée heal (I think living away from her hometown has helped, as has therapy), and also to process my own anger at my ILs. I intensely dislike the whole lot of them save my SIL, to my face they've been civil to me (I've never met FIL) and I don't think they hate me (except FIL) but I'm angry at how they failed or used my fiancée.
We've agreed on some boundaries, such as that our child will never be in MIL's unsupervised care (I've been very open with my fiancée that I cannot trust MIL to protect a child). Still, I worry (perhaps irrationally) that weddings and babies bring out the crazy in some people, and don't want to get blindsided (I also have toxic relatives of my own). I'm glad distance will make it impossible for our child to have frequent face-to-face contact with my ILs, not least because I'd worry they'd try handing the sacrificial lamb role to the next generation.
Sorry for the lenght of this - i'd be very grateful for insights, tips, advice, etc.

Are they involved in a cult? Why are terms like "chastised" and "sacrificial lamb" being used?

...and being beaten for talking back to your elder sounds concerning.

If you are concerned about your IL's influence on your child, you should not drop your child off for baby-sitting with them, but should stick arond to keep an eye on what they say.

They're not in a cult, they're pretty middle-of-the-road religious - I've seen the least favored child in other boards being referred to as the "scapegoat" or "sacrificial lamb" which is why i used the terms.
I agree, i definitely will not have any of my ILs babysitting unsupervised, thanks.

Is she in therapy? You can try all you want to help her but she needs a professional objective person.

Yes, she is in therapy (mainly CBT) and it has helped with some of the symptoms of PTSD.

There are a couple of good forums on Babycenter, DWIL (which stands for Dealing With In-Laws) and All in the Family, that provide support and advice for these kinds of issues. I recommend lurking and reading board rules (stickied at the top of the forum) before posting. DWIL posters can be very harsh and impatient when it comes to you making changes that they think you should make. AITF is a friendlier place with more room to vent.

Thanks, Middlemarcher, I'll definitely be looking into those fora, the tips are much appreciated.