Maternal Property issue , when the rightful owners are alive.

Dear sirNeed your advise on below matter. I am the son of younger sisterOur maternal grandfather passed away without writing the Will. He had 3 daughters only . Out of which the son of eldest daughter reside with grandparents from last 50 years and he has never shown respect . No adoptions is there. He is even not letting us to enter the home and misbehaving with other 2 daughters always and blackmailing to sign the NOC. he is also trying to get the succession letter illegally which he is not eligible forThe elder sister being his mother she is forcing the other 2 sisters to leave there subjective stake in the property stating he is residing from long. nnWe need to know under HSA 2005 and 2020 , if one of the daughter is supporting his son in wrong deed. Can other 2 daughters can claim there stake in the propertyIf we file the case , what all things we should keep an eye and procedure.