What's the point in defining yourself and to who? Clearly you already know who you are and no matter what you tell people they will always see you they way they want to see you.
I say them think whatever they want. Being a mother doesn't tie to just biology. You have raised your siblings children at some point. And you still guide other children as as they grow.
The problem is we have become so accustom to labels that we feel pressured to belong somewhere.
As far as I'm concerned labels only put us in a box. Humans have a complex nature. We have different personalities and can identify as different things in different situations. So why limit yourself?
The truth is you are a mother, a sister, an aunt, a grandmother and in the future great grandmother. All of these titles are linked to the female gender and age.