I'm feel sorry for your boys, OP. I have read all of your posts and want to give you a hug.
Truth is, I've been there so I know how it feels. DP and I have 2 together, I have 2 from previous, he has 1 from previous.
His son from a previous relationship is the golden grandchild, blatant favourtism and the in laws don't even bother to hide it. They have openly said they favour him because his dad (my DP) 'abandoned' him when he ended the relationship with his ex. So to them its justified. That will never ever change and I've had to accept it.
Anyway, regarding presents. They used to make it very obvious and my kids were getting to an age where they noticed how different they were being treated even after all these years. I could go into so much more examples but feel it would be too outing. I have since told them, simply, you either buy for all or none at all. If they do buy things for bio grandkids and not for the others, I refuse to accept it. After a few times of doing this they got the hint.
DP took a while to get on board, he's a total pushover and, like yours, will happily blame me for things in front of them to get himself off the hook. I put my foot down and told him I would walk away if he didn't stand up for our family - and I meant it. After years of their behaviour, he finally did.
Your DP is allowing this to continue. Your in laws sound vile but he is condoning their behaviour. He should have refused the bag of Easter things with a simple "buy for all or none". Its him that's the problem here.
I feel a weight has been lifted, I no longer have anything to do with my in laws, DP and the kids still see them every few weeks but it was making me ill and I couldn't forgive the things they did, so I removed myself and haven't seen them since before lockdown. But DP is now firmly in my corner which is really all I needed.