Don 't tell Dan, if you want to keep it a secret. the guy' s a blabbermouth.
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.
See if yu can keep your brother—who is a blabbermouth—to keep this still.
I'm such a blabbermouth that I couldn't keep the news from friends and coworkers.