Op I think there are a lot of very hasty comments on here telling you your relationship is over LTB he's a coward etc.
I don't think that's where this situation necessarily is and your post doesn't give enough information for anyone to tell you to do that. I think this mess is ultimately borne out of grief and hurt and we all know there's no correct timeline to that.
I think it's bound to be very emotional for the bride preparing for her wedding without her mum there. I'm not close at all to my mum and I remember feeling really surprised at how triggering getting married was because I wanted to involve her in more than I could. So I can't even imagine how hard it would be if you've lost your mum a couple of years prior. So having her mum's relatives there (her relatives) might be especially important to her because of the connection to her mum. That's massive.
I also think the relatives are hugely out of order, I think it's an awful thing they've done to put this on the bride and by default on your partner. However, I can only imagine how big their grief must be to think this is in any way acceptable to do. Or perhaps they are doing it to try and protect another relative who is struggling like perhaps brides granny who's not coping well? They've been able to avoid meeting you this long my guess is someone has had their head in the sand and meeting you will make their loss even more real for them.
I think that your partners role in this situation needs to be to his daughter first and foremost. I think that planning your wedding without your mum, then having this put on you which is just awful can only be topped by your dad doubling down because then what are the consequences to that? Does the brides mums side of the family not attend? Does her grandparents not attend her wedding? Does her dad not attend her wedding?
I think I would be encouraging your partner to reach out to his in laws separately between now and the wedding to try and bridge the gap for his dds sake without throwing you under it. But equally I think as his partner I'd be prepared to step back in this instance to help him support his dd and I wouldn't be putting him in a position where he has to choose. I would be telling him how this makes you feel but I'd be trying to work with him in navigating it because it's a total shit show at a highly emotional time and he's trying to do what's best for his dd in the mouth of her wedding like a good dad should. There's no win for your partner or his dd in this. To me, resenting him for this kind of means the relatives have won by driving a wedge into what's presumably an otherwise good relationship.
I'd do as another poster suggested if it can't be resolved and take yourself for a treat day instead. I would sincerely hope that a reasonable conversation between your partner and whoever is behind this could sort it out though. I feel deeply sorry for the bride in this and I'd reach out to her as well op. I'm sure she doesn't want to have to choose between you either.