The behavior is not uncommon for children to show curiosity in their genitalia and that of others. Some children even engage in sexualized play. The behavior can be something that she is mimicking (that is only a speculation)
If her mother is blaming her behavior on the visits at your home, she may have an agenda of her own to be placing this sort of blame. Instead, she should be concerned as to whether or not the behavior was observed by her daughter either on TV, day care, play ground etc.
If there are concerns about the child's safety, it would be best for her bio father to initiate this.