Eviction rights of a family member without a lease?

My grandmother is now in the hospital. My sister and her boys have been living with her for the past 2 1/2yrs since she is 89 to help out. It is a co-op type townhouse when you vacate you receive $4,000 and it goes back to the association. My grandmother's sister has Power of Attorney and has let us know she is getting rid of the place and my sister and her boys have to vacate at the end of the month. Does my sister have any rights to the place since my grandmother wanted her to stay there once she passed and what can we do to ensure the place stays in the family?