I would stick with the names Jenny and Grandpa.
I'd call the baby by their given name and the biological mum (and partner) by theirs.
This makes life so much simpler.
I think these days lots of children have blended families.
She can know her mother is her mother and still call her by her given name.
Our children always called us by our first name. They knew who we were even as little ones.
The alternative is "Mummy Jenny" if there needs to be a title as well as a name.
Her sister (surely) only needs a name.
Orchids and other lovely plants that don’t need a lot of attention



