My mother has a strained relationship with her family, particularly her mother. As children, we only met my mother's side of the family on rare occasions and now that my siblings and I are grown, I don't believe my mother is in any kind of communication with her mother or brother. I don't really know much about why they don't get along but as far as I know it's not over some particular incident. I know some general points, but I'll keep them to myself unless it proves important for the sake of this question.
Anyway, I have never had any strongly negative experiences with my grandmother. My wife and I have not shut her out of our lives and she has met our children, although we do not live close to each other and don't really make a specific point to visit each other.
Since my grandmother now has somewhat of a relationship with us, she would like me to help her get in touch with my mother. I asked my mother whether it would be ok for me to share her phone number or address (presumably to send letters, etc. I don't think she would visit unannounced) with my grandmother and she told me not to do so. I intend to honor that, but I'm trying to figure out how to word the email to my grandmother politely. How can I communicate that my mother does not want me to share her contact information without making my grandmother upset at me for refusing her request?