It's not your fault. You are in no way in the wrong. Your sisters children clearly have unsupervised access to the Internet to be able to have secret Facebook profiles...that's her mistake, not yours I also would have assumed initially that she was aware of it, and it's not like days and days passed between you finding out and her being informed. I've no advice on what to do in terms of the tension between you two, but I totally understand that awful feeling of going to bed and waking up with an unresolved issue.