I think I would have spoken to my aunt about it before I divorced her, but it's your family and you have the right to make whatever choice you want. I'm curious though, does your aunt and other family members have pictures of your daughter displayed in their home? Do you make them take them down if they have say, the cable guy, coming over? What about other people you've never met? What if your cousin's girlfriend had sent you a note, through the mail, saying that she saw a picture of your daughter at your aunt's house and thought she was beautiful. Would you still be so angry?
I guess my point is you ultimately have no control over other people's actions. My mom carries around pictures of her grandsons and will show them to anyone because she's proud. No one was being malicious in this situation, she was complimenting your beautiful family. I think I would have been a little freaked to get a message from my cousin's partner, but I probably would have spoken to my aunt about why she's letting a someone use her account before I divorced her.