WP is open to anyone who obeys the basic rules of conduct here, including your brother, so the best you can do is:
1. Don't use a username he would know and don't tell him what it is.
2. Put down a location that is not specific to where you are. I believe I am using the Twilight Zone. You are welcome to join me there or to use some other non-real location. You can also invent a location.
3. Don't put anything really identifying in your posts.
4. Don't pick an avatar that he knows you like, and don't tell him the one you use in WP.
I am using a pix from WP's own collection, but it is one that up to now hasn't been popular with others. Got a real jolt a little while ago today, though, when I saw that avatar here, checked the poster, and it wasn't me. Apparently, someone else has now picked it, too. Eh. I'm still satisfied with the pix, and it's too much trouble to change, though I can think of several pix in my desktop collection that might do.
Glad I don't have your problem anymore. My younger brother is approx 7 & a half years younger than me, and he used to be a real pest as a kid and teen, but we are both decades older now and don't live together anymore, either.
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau