DD has 3 grandmothers & 2 grandfathers:
Step-mother: Oma S(her name)
My Mother: Nana
DH's Mother: Oma (to some older grandkids who are now estranged from the family, she was referred to as Mamma because she didn't like the 'older' references but now being older she's ok with it).
DH's Father: Opa
My Father: Grandad or 'Fat Boy' - family joke. DD is learning that one nicely :)
DD's great grandmother get's referred to as GG.
You could try different language pronounciations - I'm sure there's plenty. Here I just googled some:
Finnish - Mummo
Danish/Sweedish - Mormor
Irish - Maimeo (MA-mo) or Morai (MO-REE)
