When you said you were relieved to know that therapists sometimes do that, it made me curious. Why would that make you feel relieved, unless you hope this is the therapist's mistake, which would make you feel that your brother has no legitimate reason to cut off contact with your folks? The way you word things makes me think you have already made up your mind that your brother has no good reason for what he is doing. This statement also made me curious...

I can contact him whenever I want but there is not much I can without choosing a side...

Why must you choose a side? Can't you just be a good listener and be supportive without either agreeing with what your brother says or defending your parents?