First of all yo have not stated that who the seller is ?
Assuming the vendor as grandson of the original owner and he acquired the property vide a registered gift deed, the vendor is considered to have marketable title to the property proposed to be sold as an absolute owner.
The self acquired property actual devolved on the legal owners of the original owner upon his death.
In that one of the share holder relinquished the rights i the property in favor of the other share holder, which is very much valid legally.
Now the inheritor becomes the absolute owner of the property hence he can very well transfer the property by executing a registered gift deed in favor of his son.
after accepting the gift deed, the son acts upon it and has become the absolute owner of the property
In the above context the vendor who has absolute title to the property need not take the consent of his siblings because nobody has a share or right in it.
On further aspects you can take a legal opinion from a local lawyer before proceeding with the purchase.