There's a backstory. It's long. Main points: P's sister has been unreliable and hard work since P and I first met, including cancelling on us last minute for Christmas because she had a better offer. She gave us zero support when we had a baby despite so much talk and writing said baby letters about how close an aunt she would be(!!!). The biggest issue is that I I feel like she facilitated P in leaving us (he's back now - he had a breakdown) when baby was 8 months old (by giving him someone to stay) and not asking once if I was ok... but overall we are very different people. I find her very intense, needlessly emotional, and our politics are very different, but she's not a bad person.
For balance, I haven't been an angel. I called her some choice terms once (to her face) after P left. I was drunk (not something I make a habit of, I don't drink on the whole), she was there, I was full of hormones and grief after he left as baby was still small, and I said things I shouldn't have. I own that, it was bad behaviour, but I'm not actually sorry because it was a long time ago and for points mentioned above. I also tend to forgive and forget easily with everyone else in my life, so P doesn't get why I can't or don't when it comes to her. I think it's because I've never actually had any benefit to her being in my life - I don't have any love or history to fall back on.
Anyway. Partner wants us to have a relationship, he wants us both to apologise and to make an effort together. He wants us to be able to have dinner with her and her partner and spend time as a group. He doesn't have much family and he moved to my city (where she is too) when we got serious, so he doesn't have many friends here and this would mean a lot to him.
I think that me being civil and facilitating our son seeing her - and of course never bad mouthing her and always being pleasant when we do see each other - is enough, because a) anything else would be fake/forced and b) I gave her a big chance early on and I think she blew it with the Christmas thing. Maybe a closer relationship will grow in time, but I'm not there yet.
Well done if you got this far. AIBU? Issues with P are whole other post,