You can file for a restraining order. Also, filing a false police report is a crime. Keep track of everything. And protect your boyfriend by never leaving him alone with your niece until this is all over. It's sad, but people have been convicted of child sexual abuse based on false allegations.
I agree with the previous answer. You should file the paperwork to seek a temporary restraining order against this woman. If the police seek to talk to you and/or your boyfriend, both of you should hire attorneys to help you deal with the situation. Neither you nor your boyfriend should speak to the police without your attorneys present.
Juan C. Garcia, Jr.
GARCIA LAW OFFICE, LLC