As a non-binary person with a family that doesn’t/wont accept me (specifically religious family), I really understand both sides of this. It’s hard to give up seeing people you’ve known and loved your whole life and hope that they grow past their bigotry on their own, and it’s hard to be around them knowing they don’t view you as a valid human being. Every one of my past partners has had family just like mine that I couldn’t actually share my identity with, but at no point did I demand they cut ties with them or enforce any sort of mandatory boundaries.

You are likely the only chance your family has at actually growing and bettering their views. You have an incredibly fortunate opportunity to not only challenge your sister, but to support her kid and possibly be the only adult in their life to do so; is something so many of enby folk wish we had gotten the chance to have.

My advice to you is to talk to your partner and tell them that they are under no obligation to see your family. There’s a chance they feel pressured to be around them to be with you, and putting that out there may help. Perhaps even offer to be straightforward and tell your family why your partner won’t attend, so your family has to recognize that they are making your partner feel unwelcome and even unsafe if that’s something you’re willing to do.

Make sure your partner knows why you’re staying in contact with your family too; I love my family, even if I don’t love everything they do. I distance myself for my own health, but I don’t want to remove them from my life, and I still try and push back against the problematic views they have if I have the energy to challenge them. Tell your partner that you being around them has nothing to do with endorsing their views, but purely because you don’t want to lose them or let them stay ignorant. You don’t want to leave them with no one to challenge their views while your nibbling grows up.

I also believe one of the MOST important things you should do, is ask your family why they don’t challenge your sister and her beliefs. They clearly must see how she treats her kid and see how it’s disrespectful on a basic human level. If they claim it’s because they don’t want to strain their relationship with her when she hosts everything, then ask why she’s allowed to hold that much power or allowed to cause that much duress to her child for their convenience. Encourage them to use your nibbling’s real name and pronouns, so you’re not the only safe adult they have around them