1) take search in sub registrar office and obtain copy of regd sale deed by which property was purchased by your mother
2) your mother should file suit to direct son and daughter in law to vacate portion of house in their possession
3) also seek permanent injunction straining them from disturbing their possession of house
4) record all abuses by son and daughter in law
5) install CCTV cameras in house
6) your mother can also file DV case against son and dIL and seek protection order and compensation for mental torture undergone by them
7) she can also file police complaint against son and DIL for continuous harassment