Can somebody explain why it is "muñī" and not "muñi"?
There is no separate "and". Instead the second word is modified. So "muñi" means "mothers" while "muñī" means "and the mothers".
So if we say "kepi se muñi" right ? Because I think "se" means "and"
No, you must not put "se" because it is already in the word "muñī".