The reality is that your brother and your mother are responsible for their own lives. They are adults and should not be depending on you for their happiness or well-being. They can learn to take care of themselves, and you don't need to feel guilty for forcing them to start acting like the adults they are.

With that said, you don't need to change those things to benefit from therapy. There's a lot you can do to change yourself and your own thoughts and feelings. Also, could you consider medicine for the depression problems?