1) 3 sons dont have any right on grand father property .
2) on demise of grand father his 2 daughters are the legal heirs .
3)if your mother has signed gift deed wherein she transferred property in name of your cousin brother then your cousin brother would be absolute owner of the property .
4) was possession of property delivered to cousin brother ? please clarify
5) contact a local lawyer and obtain copy of regd document wherein your mother transferred property in favour of your cousin brother
6) then move court for setting aside gift deed/sale deed on account of fraud.
7) your mother will have to satisfy court what she was doing for period of 7 years