If he made a will you have no rights unless he chose to leave you something.
If he didn't leave a will then you as his child are definitely entitled to a share or even all of his estate. You share it with any other children, assuming there is no widow. If any child has died, their share goes to their children. Noone else gets a look in. Parents, sisters, etc, only come into the oicture in the absence of any children.
His family are either trying it on or are genuinely ignorant of the law.
CAB I think