This land belonging to your late father, is agriculture land or residential land. Let me know this. Thereafter after you met the SP has a case been registered and a F.I.R. registered by them against these persons, if not then kindly proceed to file a Private Complaint Repot, P.C.R. at the jurisdictional Magistrate Court against these persons immediately.
Then this land at Punjab, secure it properly by fencing it and putting up a board, stating that the property belongs to Mr.________, any trespassers will be prosecuted.
Further place an advertisement in the newspaper(Local) stating that you, your mother and brother are the absolute owners of this land (Give description) absolutely and that you have not entered into any kind of agreement of sale nor executed a GPA in favour of any third parties. And anybody claiming to hold any such authority or agreement is bogus and concocted documents and that you, your mother and brother are not responsible for the same.
Kindly get in touch with a local advocate who can help you do all of these in order and secure your interest in the property.