1) it is self acquired property of grand father and not ancestral property .
2)on grand father demise each legal heir would have 1/4th share in property . ( 2sons and 2 daughters)
3) if shop is in name of grand mother and she has executed gift deed in favour of her grand son he would be absolute owner of said shop
4)please check whether registered gift deed has been made by grand mother .
5) grand daughters have no share in grand parents property .
6) if grand mother had executed gift deed your father can move court to set aside gift deed as your grand mother had no income and it was bought by grand father in her name for benefit of joint family