To be innocent and identified as guilty is, perhaps, one of the greatest difficulties there is. First, do not admit to something you are not responsible for doing. This helps no one and isn’t what is needed to help your niece or your family.
Secondly, I would talk to a forensic psychologist or clinical psychologist with a great deal of experience in these matters. He or she should be consulted directly about this as you will, most likely, need both individual counseling, and perhaps, a family intervention.
Do not bow to the pressure. As you are clear about your innocence, put the emphasis on your own growth, development, and self care. This may include legal consultation about your rights in preventing others talking about you in a way that damages your reputation.