Why is it so wrong to be honest?

My mom typically asks me when I'm doing, I respond "Watching My little pony" or "Playing Minecraft" sometimes "Watching netflix"
My brother responds to her with "Nothing"(he is playing Minecraft or Wizard101)

When I asked why he does that he said that it wasn't my business and I shouldn't tell my mom everything I do

I'm having mixed messages
(My brother is 18, I'm 14)

Candor is fine, so long as it's tempered with discretion. IOW, honor is the best of both of those qualities. I can never trust secretive people, makes me skeptical about their motives.

You are in the right be honest and respectful to your mom

That's great that your so honest-I'm sure your mom really appreciates it. As for your brother, he's an 18 year old guy. What can you expect?