Land registration

If the land is still in the name of your grandfather,then how the land patta can be in favour of your father? Kindly specify how your father got that land patta because until and unless you clarify the fact properly,it becomes difficult to advice.

If the property belongs to your grandfather, how he acquired, and when he has expired are basic facts.

If your GF has acquired it as self-acquired property-Your father is just a co-sharer with his sisters unless he has executed a will in favour of your father alone.

If your GF has acquired ancestral and if he has expired after 2005 then your father's sisters are having equal rights.

Denying legal rights and giving acquired property to his son alone without giving to co-sharers who are legally entitled amounts to fraud.

Contact a local advocate and settle the issue at the earliest, and if your paternal aunts' are eligible for a share, your father can not deny their share.

If the property belongs to your grndfather then your aunts are the shareholders 

The property belonged to the grandfather and on his death, property is jointly vested with all the legal heirs viz. the children and their mother(if alive). Your father can convey his share in the joint property to anyone, at his discretion, during his lifetime. 

It is of no consequence that patta is in yours father's name. Such ministerial acts, right or wrong, cannot divest divest other joint holders of their right, title and interst in the property.

I agree with experts, relvant information are missing from your post. Incomplete information can not lead to form an opinion and oblige.