Husband deserted children and my sister

Hi, My sister had some compatibility issues with her Husband who were married in 1999 December. He was very mean in his behavior most of the time. She has daughters who are 19 and 14 now. Since she was not able to withstand his mean behavior towards her and childrenshe came back to parents house in 2017 December after which he never tried to contact her or children and carrying on with his life. Her in-laws are of much help to her giving confidence and taking care of her and children all throughout with education and other needs. Now we came to know the house my sisters husband was having (which was bought in 2004 by him) was sold and advance taken and registration not yet done to the buying party. All throughout we were thinking he will realize and comeback but things are looking to take a different turn. He is not in any steady job now. Need your valuable advice on what can be done by my sister to safeguard her Children's interests. She is in a philosophical mode and wanted to let go. Please help me in understanding what best options we have to safeguard my nieces right to property of their father.