Why is that your responsibility? I say you politely tell your Aunt that you've chosen a major of your interest and recommend that her son talk to a professional as you are unequipped to be helpful in these circumstances; a study buddy will not help with depression, but a therapist can. She can have her feelings about this and you can acknowledge and validate them while still holding strong to your boundaries in a firm but respectful way.

So? Unless she's paying your tuition she doesn't get a say in your education.

Just ignore her. It’s a stupid, thoughtless, and selfish request.

So say no. I'm not really understanding why that's a big deal.

Her son is probably depressed because of the kind of mother he has.

I'm getting that you're in uni and have been accepted for a Masters. You might have bad family members, but compared to most people, your life is great. Also, the bad family members aren't your parents or siblings, so why are they your problem?

Looks like they are looking for someone to blame when he inevitably fails.. Run like the wind.