Nice
Why not "Mera"?
If you read the Tips and Tricks section of this lesson (not currently available on mobile app, only website), it disrespectful to use the singular form to address family elders, hence 'mere' is used.
Because to Respect your elder you say "mere" instead of "mera"
awww, cute relationship:)
Hey! What about My father's father but friend? Fine.....