I think that’s why the writers made her so soft spoken. When you hear
everything, you would know everyone’s secrets. Hard to know when you should be saying things, and when you shouldn’t. She must’ve known Bruno didn’t want to be found, and kept it to herself.
During the Bruno song she even says
“It's a heavy lift with a gift so humbling
Always left Abuela and the family fumbling
Grappling with prophecies they couldn't understand
Do you understand?”
Which indicates to me she understands Bruno’s plight, and wants to let him stay “missing”.
I haven’t figured out yet why she couldn’t help but share what she knew at the dinner party though.