In this particular context, being a mother of someone, and being a mother to them can be quite different.
For example, you might say of a step-parent, foster parent or anyone else that has raised you that they are 'like a mother to you'.
That's not to say that step-parents or similar cannot legimitely say that they are a parent 'of' their step-children - you'll find that people make their own rules of language on a case-by-case basis there. But 'Yorkies', as hopefully you know, are dogs. Their 'parent' (or 'pawrent' as I'm hearing more and more) definitely didn't give birth to them, but she can be a mother to them.