Don 't tell Dan, if you want to keep it a secret. the guy' s a blabbermouth.
See if yu can keep your brother—who is a blabbermouth—to keep this still.
EXAMPLE: My little sister is a blabbermouth who always ruins surprises by telling everyone what is going to happen, so no one in our family ever tells her about surprise parties.
I'm such a blabbermouth that I couldn't keep the news from friends and coworkers.
