1) your mother should issue legal notice to her sister in law to vacate the property
2) sister in law is only gratuitous licensee and has no rights on the house
3) No one acquires title to the property if he or she was allowed to stay in the premises gratuitously. Even by long possession of years or decades such person would not acquire any right or interest in the said property.
4)file suit for eviction of SIL and her family
5) The protection of the court can only be granted or extended to the person who has valid, subsisting rent agreement, lease agreement or license agreement in his favour.
6) since your mother is a senior citizen seek expedited hearing of the case