If your nan is upset with your cousin that is her problem, not yours. Why should you sacrifice the start of a fresh relationship with your cousin because one member of your family thinks that it is not okay. I think that it is wonderful that your cousin have spoken to you and this could be something that will end the feud. Your decision to meet up with her is your business only, don't involve your nan, the less she knows the better she sleeps. At the end of the day you are all a family and you don't know if some day you will need each other because life is a crazy rollercoaster. Hope it works out!