Help with Invitation Writing?

Hi all
I am attempting to write my invites but struggling with the Nan's. The invites are traditional coming from my parents with mr and mrs Sam invite blank to the wedding of their daughter xx to xx. We are writing first names but Sunday names (Michael instead of Mike, Victoria not Vicki etc), what do we write in my Nan's? Mum or her first name?
Thanks in advance for helping with this headache!
Xx