So, this all happened in 2015, and it's kinda lived rent free in my head since then as it caused my in laws to hate me. So my ex wife's sister and her have a bit of a strange relationship, i don't know exactly how or why, but my ex wife's sisters has alsways been very critical and made nasty comments about her.

Its worth noting that my ex wife at the time was about a UK size 16, so she wasnt exactly big, but she was always beautiful to me, in fact, its one of the things i loved about her.

Shortly after we got engaged in 2015, my wife hung out with her sister, she had a lovely leather jacket which really looked good on her. Her sister made some sort of comment (I cant remember exactly what she said) but it was something like 'Couldn't you have found a jacket that fitted you?' or something similar that impled she was fat or should lose weight.

When she told me this, i got a bit upset, because my ex wife was clearly upset about the comment, and I said she should get an apology from her sister, to which she replied her sister would never apologise, or was unlikely to apologise (i cant remember exactly now).

I said that if she didnt apologise, i didn't want her at the engagement party, to which, while my ex wife wasnt pleased about it, she didnt seem to object.

Well, when her parents found out about this, they gave the ultimatum of either her sister comes, or they wont come, to which i said that her sister was welcome to come if she apologises about making the weight comment that upset my ex wife.

Well, to nobody's surprise, she refused to apologise, and therfore neither her, nor my ex wife's parents came to the engagement party.

So AITA for unininviting my sister in law, after the comment she made? I always thought partners were supposed to have the back of their other halves and look out for them?