I feel so sad and annoyed and I don't know if I should be, or what I can do (nothing really this year).
My DD2 has wanted a Barbie dream house for Xmas for months, she's asked Santa for it and tells everyone that's what she wants for Xmas.
I try to be mindful of the environment and often buy big items from Facebook marketplace. They're like new, means one less toy eventually added to landfill, and cheaper. So I found her a fab Dream House bundle on there that was like new and snapped it up.
I'd been asking her dad what he wanted to do for months about Xmas. Last year we split the cost of the main present and then bought a few smaller bits each for her too. This year he was very non committal and vague so I just went ahead and bought it. I scoured marketplace and ebay, spent ages looking, went across town and picked it up, hid it, etc.
Last year on Xmas day (of which he was invited to mine for the day) he declared all the toys from his relatives would be going to his house (DD goes there for tea 3 times a week). His relatives had asked me for ideas and I based these on thinking they were to stay at home, so after Xmas there were some gifts she'd been really looking forward to playing with that she could then only play with a few brief times a week.
This year when his relatives have asked for ideas I've advised them to ask her Dad as he will want them to go to his house and I don't know what she already has there.
When he knew I'd bought one he said "oh shall I give you the money or half the money". I said if you do, will you want the toy to be at your house and he said yes. 
We were in a room with DD1 and in my head I was thinking..... How do I say no without being arsey about it or pointing out how I'd been asking for ages, had spent hours searching, etc. Thankfully DD1 chimed in instantly and said "I think that's an awful idea because then DSis won't barely be able to play with it and she's super excited." (DD1 is 12 years old with zero social filter). Secretly I was so relieved she had pointed this out to her Dad and I could then say yes that's a good point maybe you choose another lovely gift.
A week later ex announced that he had bought DD2 a brand new Dream House for her to open at his house and keep there, and his mum had bought a HUGE bundle of barbies and stuff to go with it. 
MIL is awful for over buying and when we were together ex was very strict with her and tried to reign her buying in slightly. It's lovely she's generous but it's over the top minimalism and opposite of my views and how I want my kids to think about Xmas.
So now DD2's main present from Santa has been fully paid for from me, so I haven't bought her (in her eyes) a big present, but Daddy has bought her a bigger better shinier version of her santa present, and granny has bought her tonnes to go with it. And they've then also bought lots of other bits too.
I feel annoyed with ex. I feel sad I'm going to be spending all future xmases like this. Anything I buy and take time to get and make special they will just out buy me and go one better. DD1 has special needs and I'm her carer, I'll never have the income to match them and even then I wouldn't want to. I don't want them spoilt or materialistic, and I don't want to enter into an ever increasing spending contest.