There certainly are a lot of concerns going on with the people in your family. And your uncle and aunt want you to do more - maybe they're hoping for your support to help your brother? Are they nearby? In order to communicate about your boundaries, could you let your uncle know that you want the frequency of phone calls to change - maybe you're agreeable to once a week phone calls? What frequency of phone calls would work ok for you? Could you state "please don't phone me every day". Figure out in your own mind what you feel comfortable with, and then inform them.