OP, I have flaky inlaws and totally understand your exasperation. For years I tied myself in knots trying to maintain a relationship with them for the sake of my kids, who adored them. We/the kids were always way down on the priority list.
We ended up falling out over a deliberate snub by them, which they insisted wasn't a snub. If the relationship had been otherwise ok we'd have got past it. However, it really opened my eyes to how one sided the relationship was and how disinterested they were in us.
We ended up going NC. There wasn't a big scene, I just waited for them to contact us. That was about 3 years ago. Still waiting.
To be honest, it's made life much simpler and less stressful. The kids were really sad at the time and missed them, but they asked about them less and less and now they don't feature in their conversations. We (OH and I) decided that it was less hurtful for the kids just not to have contact, rather than constantly being let down (final straw was when my eldest asked if she'd done something wrong because SMIL didn't want to see her any more
.)
In your shoes I would stop making the effort with your brother, and let it die down. Reset your relationship with your mum so it is more equal in terms of travelling, Christmas etc and be firm. Your time with your kids counts too and dragging them on a 150 mile round trip every other weekend isn't fun for them. It will only get trickier as everyone gets older, so it's a good time to encourage her to do maybe 1 in 3 trips down to you now.
When I was struggling with the toxic inlaw situation someone wise reminded me that we cannot change other people's behaviour, only our own and our reaction to it. I was giving them far too much head space and constantly questioning their motives, much like you are doing here. For your own happiness you have to move on.
Re the parties, I wouldn't go, and offer to see them for a joint birthday celebration in between the dates when you are at your mum's. You could maybe take your children and brother's out (without brother), make it a joint celebration for them all.
I also agree with other posters, that as the children get older it becomes more difficult to involve cousins - age, gender, distance all play a part. Find other ways to be involved in their lives, perhaps the cousins can start emailing or facetiming, or you can arrange meets at granny's house.
I think you've had a hard time on here; until you've been in this situation you don't appreciate how it's the little things that push you over the edge. It's clearly not about the party, it's the tip of the iceberg.