It's your baby and your decision. It irritates me so much that people make negative comments or try and suggest other names or change your mind about your own babies name. I understand not wanting names that run together as we are having the same problem ourselves at the moment. (One of the names on our list ends in M and the middle name starts with an M - it just doesn't sound right!)
Do what makes you happy. Your mother will get over it eventually and her name will be Hayley not Nicole so there is no reason for her to think of her cousin really.