If it's any consolation, this is exactly what happened with me and my sibling. 8 year age gap, family in better financial situation by the time sibling was coming up to secondary. I did really well at a mixed state school and got into a good uni, sibling was starting to struggle with focus at state primary and didn't get into any of the good state comps nearby, so our parents decided to put them into private. They went on to achieve excellent grades at GCSE and A Level, as well as a first at university.
I've not been jealous or upset at any point as I think our parents' reasoning made sense, and I've not lost out on anything. Sibling needed the extra boost at school age, I didn't. Also, as you are saying you'll do, our parents have tried to "make up for it" by covering some of my milestone expenses as a young adult even though I never expected them to "balance the spend out" because I didn't feel like anything was unfair. But it made them feel better to be what they thought of as fair to both children.
Edited to add that my sibling also got financial support from our parents towards a house purchase but they felt it evened out because sibling moved away from the South East so had a much, much cheaper first time purchase. As far as I'm aware, sibling doesn't resent parents for getting a "smaller share" of deposit contributions, they are quite level headed (probably thanks to the good private school they went to?!) and appreciate that any contribution was given at all.
Similarly, my DH's parents have recently insisted on giving DH a lump sum. They contributed to our first property purchase about 8 years ago as well as DH's sister's first purchase recently. But at the time we bought vs when DH's sister bought, they were more financially stable, so it seems they felt bad about how "little" they contributed to DH. DH argued that they didn't need to (I agree with him) because any help was appreciated back then and we are l ourselves are more financially stable because of where our careers are now and the fact that we are in our 30s, but they still insisted for "fairness."