'Who' versus 'what' when talking about relationships between two people

'What...' is more common than 'Who...'

What in this context does not refer directly to her. It refers to the basis or reason for your knowing each other. The question asks what it is that she does that results in your knowing each other (e.g. she goes to the same swimming pool as me, she is my accountant...).

Who refers to her directly. The question asks her relationship to you (e.g. sister, mother, mother's friend, best friend, class mate...).

Clearly, there can be some overlap. But, taking your first example...

Who is she to you? She is my grandmother.

What is she to you? One of the most important people in my life.