1) your parents should move court and seek permanent injunction directing son and daughter in law to vacate the residential premises
2)your parents should obtain injunction order to restrain son and daughter in law agents, representatives, assignees,associates etc. from creating disturbance in the peaceful possession and occupation of the parents self-acquired property
3) your father being a senior citizen can also move senior citizen tribunal for necessary reliefs
4) we have number of lawyers on this site from New Delhi . you can contact Mr Ashish Davesar from kaaanon.com . contact details can be obtained from administration
5) legal fees vary . lawyer whom you engage will inform you the legal fees for drafting and filing suit in court