diet grandfather property can be gifted by grand mother and father to his grandson. it will legal or illegal. can any other II grandson or granddaughter challenge it.

Who else are other legal representatives of your grandfather to get the property in succession ?

It appears from the post that for intestate property of your grandfather, his wife (your grandmother) and his only child (son /your father) are only legal heirs.  If they wanted to gift it to you, they may register a gift deed which is in order.   If you do not wish to face problems, let your brother contribute his signature as a witness to that gift deed.  Then he may not be having a chance to challenge the gift deed.

Yes, on the death of the grandfather, the property is vested with the grandmother and father. They can , during their lifetime, dispose of the property at their discretion. But why this hurry in excluding your siblings?