I can't believe some people. I am so cross on ypur behalf ypur poor daughter :(
I have a daughter whose father has no involvement and a new husband and baby. Dd is 7 and baby is 6 months. DH has been in DDs life since she was 2. I can't imagine my husband ever doing anything like this or his family. Him and his family do not differentiate at all. It's always just daughter/granddaughter/cousin etc. And if one gets they all get.
I think I am most cross at your husband tbh, regardless of what he says he clearly does treat them/view them differently. Your youngest would be going to private school over my dead body if eldest wasn't given the same opportunity.
It was my DDs birthday recently and BIL and SIL forgot. My MIL and FIL were livid!
That is how family should be. No child should be made to feel like a second class citizen in their own family.
I hope this doesn't come across as me shouting at you OP, you have dealt with everything very well and amicably.