Can sisters force a sell of a home after death of mom

My husbands sisters are trying to force us to sell the home we live in with the mother after she passes. We have lived here for almost 11 years and have paid all expenses for the home including utilities, property taxes and repairs. I am listed on all the bills and the home owners insurance is in our name. We were promised that if we lived here and took care of the mom and cared for the house we could purchase the home after she passes for the amount due on the reverse mortgage that was taken out about 20 years ago. The sisters have known about this agreement and never expressed any desire to buy the home or have said anything until this last week where they decided they were “entitled” to an inheritance of 1/3 of the house. They keep pushing my mother in law to sell and it is causing a lot of stress on her and she has told us multiple time she wants us to have the right to buy the house and if we don't then it could be sold and split. What can we do and how can we fight against them?