Rights for co trustees of an estate

My sister died. My niece wanted to buy the house which my parents bought for my sister. She did not have the money so my mother put a lien on the house for 40,000. My niece and her husband both signed the document and it was filled with the proper authorities. My mother has since passed away and my sister and I are co-trustees of her estate. We contacted my niece to try and settle this and she replied saying that my mother never intended to collect on this. Do my sister and I have legal rights to this money and can we make her sell her house to pay back the 40,000 dollars?