Thanks for all the feedback.
WRT the backstory, it is long and boring, but I'll try to make it as short as possible...
My SM and I have never particularly got on, but only saw each other once or twice a year. 4 years ago, after I split with ExH (for EA and DV reasons), stepmother carried on seeing him a few times. Which is bad enough, but she lives 2.5 hours away from him and had made special trips to have dinner with him. On the last trip, she promised him some valuable items which had been awarded to me in the divorce but were being stored at her and Dad's house. I only found out because ExH phoned me up to brag about being on route to collect them.
At that point, phoned my dad and went apoplectic. He asked me to email him a copy of the court order awarding me the items before he would promise not to hand them over to ExH, which I did
I said I wasn't going to see SM again until she apologised for this. Dad stood by her and said it was her 'human right' to meet up with whoever she wanted to, and she didn't see that she had done anything wrong and therefore had nothing to apologise for. All he would say was that it was 'ill advised, but not malicious' and I was making a big deal out of nothing.
He said she thought she was doing the right thing promising him the items, because he had shown her an email in which I appeared to say he could have them. I have never seen this alleged email.
Fast forward 3 years, and I've avoided seeing her, until a big family party this summer.
My son had been stillborn 8 weeks earlier, and it was obviously hugely still raw. We were seated at opposite ends of a table of 20+ people, but while we were eating, she stood up to make a 'big announcement' that her daughter was expecting a baby, and she was over the moon that this christmas would be extra special because there would be a baby there. She then offered to show people a scan photo. Bear in mind that the only people at this table who have met her daughter are my dad, me and my brother...
At this point, I said something to my dad along the lines of 'Are you seeing what I am seeing? And you aren't doing anything about it?' to which he replied to should stop making a fuss and trying to ruin the occasion. (I think I was beyond restrained...)
After this, I emailed my dad to say DH and I were stunned beyond belief by her behaviour and him standing by doing nothing, and also said I was very disappointed by his behaviour after losing our son, where it took him several days to get in contact, and even then, there was no offer of support.
He replied to my email saying, among other things, that he and my mother had experienced a number of miscarriages but had just got on with it, so him and SM didn't understand why DH and I had to make such a big fuss and organise a cremation (we didn't organise it. The hospital did).
I replied to this saying I was even more disgusted, and until both of them were prepared to apologise to both of us, I wanted nothing more to do with them. If they were prepared to apologise, they could send an email with a time and place. Until the Christmas presents, I hadn't heard anything from him
I know reading this back, I should have cut all contact at the point of the initial fallout, but it was a few months before my wedding to DH, and I wanted some sort of illusion of family there, especially when it would be obvious my mother wasn't there. SM wasn't invited, obviously