Is it necessary to file for probate?

My mother recently passed and I am trying to determine if I need to file for probate. The only asset she has is her house which is reversed mortgaged. She owes 150K for the mortgage and we think it could be sold for 200K. However, she has about 50K in debt. My sister lived in the house with my mother and wants to understand her options: If she wants to buy the house, do we need to go through probate or can this be worked out directly with the mortgage company? If she chooses to not buy the house, how long can she legally stay in the house? On another note, are we legally required to file for probate? Since her debts would eat up any equity in the property.