ESH

You are 100% not entitled to his money. You are not married and you took out the loans to further your career prospects. Boyfriend is 0% responsible to pay them off. Would it have been super nice of him? Sure, but it’s not a moral issue. His sister is literally his sister and he’s know her all her life.

He sucks because why is everything split 50/50 when he makes 3x as much as you? By splitting it 50/5O dollar wise versus ratio wise, he’s able to save much more than you are ratio wise. After 3 years of dating and now living together I’m surprised you both haven’t sat down and talked more about how financially in contribution you can make it more equitably similar. That may mean thinking beyond matching a dollar to a dollar formula given the huge salary delta.