I am a  nonnie. Does this mean that the changes my mom is probably also a nonnie (I have the suspiscion she is)? Or is nonnieness not hereditary?

it is hereditary... but one of your parents  might only be a carrier of the nonnie gene, but musn't be a real nonnie in life ...also here depends what grandparents have been....

I'm nearly sure that both of my parents were nonnies....

You have to inherit nonnie from both parents, but like Isa said it could have been resessive in one or both parents.  I am a nonnie but my son is a secretor, so he is a carrier.  If he has a child with a woman who carries the nonnie gene then he may have a nonnie child.

Sounds like nonniescense to me!