Throw-away account. And this is gonna be long, I apologize. I (25F) have been with husband (24M) for three years, married for 6 months. I had what would be described as a pretty "normal" upbringing, two married parents, very little drama, only ever moved houses twice and all within the same zip code, so I've been blessed with stability.
My husband, on the other hand, has had a much harder upbringing. Now most of this isn't my story to tell and I'm gonna keep it pretty vague. His father, who he is no contact with, made him and his three siblings upbringings very traumatic, and he moved them all over the country. His mother, who had already had a pretty traumatic life beforehand, wasn't in a position to leave with the children.
When my husband and I first got together, my (now) MIL kicked her husband out of the house, and they've been separated ever since. It's my husband's goal I never meet him, he wasn't invited to the wedding, etc. In the house is now just his mother and the two school-age siblings. Tensions are high there all the time: it's two young girls that have only known trauma and dysfunction and a mother that basically also has only known trauma, and no one there to regulate it. It's a lot of "pointing fingers" from all parties all the time, and my husband often has to play the "father figure", which leads to awkward interactions.
Throughout my husband and I's entire relationship, I've struggled with connecting with all the women in his family: his mother and his sisters. I tend to shut down around his mom (I'm autistic and can go low-verbal when I'm stressed) and I still can't see her as a safe space. I know she told people at her church that before my husband and I got married that I didn't know it yet but "I'm now hers" and she tried to say in her rehearsal dinner toast that I was replacing her role (which thankfully my husband shut down), and so I've often found myself shying away as I don't want to be involved in that dynamic. I appreciate that my husband has played a larger role in her life than a son should, and he had to step up when his father didn't, and those lines were crossed probably.
With his sisters, one is middle school age and one is late-high school age. They've both had to grow up way too fast because of their upbringing. They are not well behaved, but their mother also is not fair with them. (Again, no one here is regulated). MIL won't let the siblings go to a therapist that isn't faith-based, and so neither are in therapy and learning how to process their emotions and experiences. I think fostering a relationship with the younger sister could be easier, as she's still at the age where I could take her out to do things and invest easily in her interests. It's just hard because MIL keeps her very close-to-the-hip. But I have no ideas with the older sister. She's living the standard overworked high school life on top of the unfair things life has thrown at her, and I can't think of a way I could try and be a presence in her life without it feeling forced.
I'm asking advice from people who've been on either side of broken families or experienced traumatic life events, how you would maneuver this situation, both with my MIL and my sisters-in-law.? How could I begin to foster these relationships? I don't know if I've provided too much information, or not enough, so feel free to ask follow ups in the comments.