ñuhi should it be "our" instead of remaining "my"? otherwise why isn't it simply "ñuha muñi lirisi"?
Ñuha is an adjective, and means "my". The stem is ñuh-. Like all adjectives in High Valyrian, it agrees with the noun it modifies in case and number, which means it takes different endings depending if the noun is plural, for example. In this case muñi – "mothers" – is plural, so it takes the ending -i, which gives us ñuhi.
So ñuha and its various forms never means "our". The adjective "our" is completely different – īlva.
"It can't be lirisi, because that would mean I have more than one mother", I thought to myself. Yep, got fooled by that one.