Dealing with Neighbor Dispute

My family lives in a duplex house and our neighbor has continually tried to
bill us as the head of a "condo agency" that she set up. These bills include condo fees and payments due to water damage in her area of the house (we both experienced the same problem because the person who sold this place to us did a poor restoration job and my mother paid for the damages to our part of the house), a shovel she bought, and mats that she installed on the bottom area of the hall. For a while (3 months) we ignored her bills and her knocking on our door after she and my mother had had an argument about the situation but the situation still persists. She claims the things she is charging us for is for the good of the house as a whole but she did not consult my mother before taking action. My mother is getting quite angry because her continual harassment of us is making me feel anxious every time I hear someone coming up the stairs and she is even considering putting a door at the bottom of the stairs to keep our neighbor from approaching us. I was wondering if there was any legal venue to keep our neighbor from sticking bills in our mailbox and door for her personal expenses and I was wondering if we are legally bound to pay her despite her charges seeming to be for personal gain rather than benefiting the household. I have tried to act as a mediator between the two but they keep arguing with each other and I am getting frustrated and stressed by having to deal with this situation in my own home.