Are you projecting by any chance? Such an aggressive attitude.
My god. This isn't the end of the world. They will get to have the opportunity to put the tree up at their primary residence. While the child that lives there gets the opportunity to put their tree up. Not every single thing has to be shared and catered to. No wonder the whole thing turns into a shit show.
I'm a stepmum and my DC are also in a blended family on the other side. Their primary residence is with me and DH. I prepare for Christmas based on what I've always done. I absolutely include my SDC but I don't change things to work around them. I equally don't expect my DC to run the show on the other side - the entitlement!
Equally, my DSC are catered to at their primary residence with their Mum and her DP.
My DH and I also specifically plan activities with all our DC so there's lots of inclusion. We are all seeing a panto next weekend that DSC don't even want to go to because it's 'boring' but their Dad has insisted and I'm absolutely happy to take them with us. We are then heading into London for a shopping trip and to see all the Christmas lights with all our DC. My SDC want to shop in Central but not do the Panto. They've asked to be picked up after the Panto. DH has said absolutely not. I know they'll be grumpy at the Panto but I don't care, it doesn't affect me or my DC because we just don't register it.
Blended families are hard and so long as you are treating everyone with kindness and compassion, you'll be ok. Live your life, make your plans and keep moving ahead. If you stop and analyse and try to please everyone, you'll end up stressed, resentful and bitter.
Don't give it a second thought OP.