Hi sista,
I'm afraid I don't know if she has any legal rights there, I wouldn't have thought so seeing as she already does see the kids, I wouldn't have thought she could take you to court because she wants to see them more.
I thought the court thing would only come into the picture if you cut her off completely or if she thought here was abuse or neglect.

Sorry I'm not much help but wanted to offer you some and hope someone has the answer for you.