मेरे पिता का भाई मेरा चाचा है
why is that wrong?
It is to show respect. You respect your father, hence you say "mere" pita. You also respect his brother/your uncle, so you'd use "ke bhai" and "mere chacha"
"के "is mostly used for elders or to show respect but "का" is used for people of same age or younger than you
I'm not sure but it's probably not respectful enough, although how you would know is beyond me.
Any clarification on this anyone? I guess it is respect due to पिता hence uncle.
Why is के used here?
Same reason - honorary.