Grandma is a fleeting character...

She "appears" for only a few seconds...

It's just that Ma wanted a grandmotherly figure in the performance, because she thinks that granmothers exist to make this world a better place... Besides, Bar of Wishes is a huge fairytale! And can there be a fairytale without a grandma?

And, thus, Grandma was created...

It should be fairy-noted:

Ma loves four grandmothers...

She sees two of them once in a while. They live and thrive on two distant islands...

She never met the third one, but she's heard a lot about her...

The fourth she knew, and she played a very important part in her human life... but, then, she was gone... she faded like the stars that live for 10.000.000.000 years and then they suddenly "die"... she left... and she took with her a small, wonderful part of this world...