With all due respect he’s 17, if he wants to go to university he can. But it’s something he needs to sort himself with the support of his mother and you if you want to continue to be so invested.
He can easily go to uni he will just have to put his mothers income down and claim he is estranged from his father for funding purposes, he may have to get a part time job for spends but that isn’t the end of the world.
What dad says doesn’t matter the lad is practically an adult (legally that is) he can do what he chooses.
I’d not discount the apprenticeship quite so easily though (maybe the field I would) apprenticeships these days often attain a degree at a much lesser cost than generic university and can be easier to do/organise work and lifestyle commitments around. Perhaps this is something he could research?
I don’t think comparing a 17 year old with a 10 year old is really fair, no one will know for another 7/8 years whether your BIL will encourage uni for her or not, and you should form opinions made on assumptions, it just sounds like you have a grudge and don’t like him very much.
I can’t see how they’ve been treated differently as in unfairly more differently as there is a 7 year age gap.
The crux of it is if it’s uni his heart is set on then he can go if he so wishes, but the onus is on him to make that decision and get the ball rolling.