Is it kepī instead of kepi because it is Mothers AND fathers?
Yes. Instead of using the word "and" when listing things or describing multiple things, the last vowel (I think) becomes elongated. :)
First is mother and then father if is viceversa is wrong
The pattern I have noticed is (Young) (Old) (Action) (Link)
and usually with genders its (young girl) (young boy) (older girl) (older boy) (woman) (man) (action) (link).
Hope that helps!!
There was a sentence earlier that said 'kepi muñii kirini issi', but I like your theory.
Sometimes it is muñī and sometimes it corrects muñi. I wonder why the accents are different?