Situations like this is really difficult. I know from personal experience between my Mormon and Wiccan friends. And let me tell you this: They only get along when they have to. Their parents don't like each other and it's really frustrating.
That's why if you want to tell your parents, you have to slide in the topic very subtly. Don't be like "hey Mom and Dad, I'm going to be a Wiccan." Bring the topic up by talking about Wicca and how you feel like it's interesting or something like that. If they seem to take it well that you are interested (as in they don't totally flip out on you like my Mormon friends' parents when they found out that their child was associating with Wiccans), then you can bring up the fact that you might want to be one but not in that conversation, maybe another day. That was just to let you know if they tolerate it. When that other day comes, once again bring up the subject subtly and tell them that you want to be both because you find that both help you express yourself and all that jazz. If they flip out, I suggest not telling them about it just yet until you are absolutely serious about it.
About your brother, either get him busy with something or just don't do anything that would expose you if he's as nosy as my little sister. I usually do stuff that I want to keep private when my little sister isn't around or is preoccupied with something. You should probably do that too when it comes to your brother.
I really hope that things work out. Bets of luck!
