I imagine the family member probably just clicked on a friend suggestion for someone they once knew...and doesn't realise that any contact with anyone related to my ex is a big deal to me. But if I don't accept it I'll always wonder what it was about...
It's most likely something like this. Or she could want to contact you.
If you "friend" her, your ex will see your feeds. Do you care?
You could accept, send her a message like "hi how is it going" and see if she is trying to connect for some reason. If you don't get a response in a few days, unfriend.
What kind of relationship did you have with her?