Ask your brother if maybe he wants to draw too since he's so interested in what your doing.
As for other people when you notice them over your shoulder just open a new layer for digital, or off in the corner for traditional, and just draw some.. erm male genitalia, the cruder the better. When they ask just say "Oh I thought you wanted a show. I'm not really a performer so this is the best I can do" It's how I handled in in HS.
It's okay to be a little rude. If you explain that drawing in public is like texting in public. You can also text in public and if you catch someone reading texts over your shoulder, you would think they were rude. Put the rudeness on them. Let them know that it's rude.