I've come out as bisexual to just a few select people. I've asked the question many times about who else I should tell. What I've figured out is that unless being gay/bi "IS" who you are (then tell everyone), you only need tell those people that it would affect.

For example, my wife knew before we married since we have a sex life, she should know. However, my grown kids don't know since they are not part of my sex life (well they were to be born...). As I'm not going around bringing guys home to have sex with, it doesn't affect them directly. Similarly, telling my parents (in their 70s) wouldn't change anything for the same reasons. Telling a doctor or a therapist is fine as they may need that information for health reasons.
I guess in your case, your brother was affected directly due to your sexual orientation, then maybe that is a good reason to confide in him. If he is your brother and he loves you, he'll understand (maybe he already know).