TL;DR: My sister cheated on her husband, and I think she’s been trying to convince me to move back home to use me for money and childcare during and after the break-up and divorce.
Background:
When my (46 F) sister (39F) got pregnant, her mental health tanked her third trimester and post-partum for several months. She quit her psych meds cold turkey during the pregnancy, which contributed to her decline in mental health.
I live 200 miles away from her. For the first 6 months after my niece (4F) was born, I bought a train ticket every weekend and spent every weekend with her to support her and take care of my niece, and to be a responsible adult in the room just in case her mental health declined again.
I did this so my brother-in-law (39M) could work full time on the weekends, then stay home during the week to care for my sister and his daughter.
I also sent my sister and brother-in-law about $6000 during the first 2 years after my niece was born. I don’t expect them to pay me back because I didn’t see their financial situation improving to a point where it was reasonable to expect them to pay it back. (I was saving to buy a house at the time, so I had savings to give them.) After my savings were exhausted, I told them sorry, I’m broke, and I can’t keep giving them financial support.
Bottom line, it was a very stressful time for all of us, and both me and my brother-in-law did everything we could to rally around her.
Why I want to cut contact: She cheated on her husband, my brother-in-law. He suspected something was up based on how she was behaving, but my sister made the mistake of taking my niece to meet the dude she was cheating with, and my niece spilled the beans. My brother-in-law demanded to see her phone. He took screenshots of everything he found. He sent me some of them. She’s hooked up with multiple men, potentially without protection. It’s been going on for nearly a year. There were text messages where she talks about financially using my brother-in-law and planning to bounce once my niece was in full-time school because she’d have more income after no longer needing to pay for daycare. The text messages are so bad; there is no way for her to make excuses or talk her way out of this.
I told my brother-in-law to get a lawyer, save the evidence, and stop talking to her. I told him all communication not related to his child’s daily needs, (food, medical, shelter, education,) should go through a lawyer.
This past year, she’s been spamming me with jobs in her city, even though I’ve been clear about not wanting to move back. I suspect she wants me to move closer because she wants me to use me for childcare and money, not because she wants to spend time with me. She lives in a high COL area, and I know she can’t hack being alone financially. Our entire childhood I was parentified and forced to take care of her. I did her laundry, cooked meals, cleaned; when she got lice, I’m the one that walked to the drug store to get the shampoo and comb her hair out strand by strand. When she had seizures as a kid, I walked my ass to the hospital every day after school to spend time with her. Obviously, I’m still salty about being parentified as a child, and I don’t want to be forced into that again now that we are both adults. However, when this is all over, I want to cut ties with her to avoid being dragged down by her drama. (I would not cut ties with my brother-in-law or niece.)

NTA. Sounds like your sister has some serious undiagnosed issues. I know it’s the Internet’s most common diagnosis from non -expert strangers like myself, but she eads as highly manipulative and narcissistic. Extremely self -absorbed at the very least.
Whatever the case, it sounds like you’ve been understanding and done everything you can. I don’t know if she lays guilt trips on you or not about doing more,/not enough, but if so that would be another check in the ‘cut-off contact’ column. Good luck.
This isn’t even the half of it. She got into a fight with her husband because she started planning a solo trip to Puerto Rico, without discussing it with him, while they are struggling financially. I heard about the fight from her, and I told her she was wrong for not talking to him about wanting to take a trip first before making reservations and paying deposits.
Our mother abandoned her while she was in junior high-school, and I had to take responsibility for her because I was already 18 at the time, while I was putting myself through college. Everything she’s doing is giving me flashbacks to that time in my life.
I can understand being unhappy in a relationship, and if that’s the case I’d tell her she needs to sit down with her husband and start working out a break up plan. That was something that should have happened before fucking other dudes. I’m worried that she will ultimately abandon her child.
I appreciate your comment, thank you.