YTA

Put yourself in your brother's place. Suppose he's the rich brother and you have two children and he gives a ridiculously expensive gift to just one of them.

You offered both a salary of 11 but later offered only her a salary of twice that. All money above the original salary of 11 is effectively money you gave her for free, but not her brother. Not that this would make what you did any less unfair, but you didn't deny him this gift for being lazy or unwilling to work, as he took another job offer that compared favourably to your initial one.

Did the Instagram stuff have measurable returns and did you pay a fair market price for it, or was it just pretend work to give her more money?

It might be a lot of money, but you can set it right by admitting you were unfair and giving it to your nephew over the next few years, or paying his university loans.

if he was so worried about his children's education he should have saved up for them because he had 18 years to do so, I saved $63 000 in 3 years

His son was looking for a job at 15 and could get a loan for university. He did, in fact, prepare his children well enough to afford university, if through loans. Taking out a loan for university does not mean one is financially irresponsible or tries to live beyond one's means and you should not chastise your brother for letting his children do it. You just went and gave your niece a lot of money.

If it's so easy to save 63 000 in 3 years, do it again for your nephew as you did for your niece.