Gift deed without signature of major son

Is the land gifted by your grandfather to his adopted daughter self-acquired or ancestral?  If ancestral, your father has also right over the property and if it is self-acquired then it is his will and no one can claim it.  Your aunt is selling her land and you have no right to prevent her as it is her own property.

This is an ancestral property.

If my father has a right for share as it is ancestral, is the gift deed void and defective? Or does it mean that only one acre has been transferred effectively? Or now my father should approach civil court seeking to make gift deed ineffective? thanks.

The presumption is that the litigation/conflict is on one acre alone, which was purchased back from your paternal aunt as the 30ac land was already gifted by your GF to PA.  Be brief and focus on the vital issue.  If the one-acre was sold by you and given possession to someone long back, let the buyer who is in possession fight it out and support his case depending on such period after sale. as he must have mutated the property and obtained pass book in his name long back.

How is that the property is ancestral? Moreover, the facts posted suggests that much antagonism between the brother and the ( adopted) sister . It appears that the brother wants to ensure that the sister gets no parental property, by hook or crook. 

There are few facts here. 

1.The land of 2 acres given to her by registered gift deed is out of a big piece of 10 acres which was not sub-divided. 

2. The gift deed was signed by only grandfather when there is a major son and from a joint family.

3. Grand father got this property from his father and ancestral.

4. She sold away one acre and never claimed the remaining acre. She does not know where the land is. She never stepped or cultivated this land. We have been in possession and enjoyment. This happened for the reason that she has taken money for this land and gave up the land. This happened about 30 years ago. 

5. She got passbook, it appears, without we knowing about it. It is a news for us now receiving an enquiry notice from MRO.

6. Unfortunately it is quite common in some parts of India giving gift and buying it back. For so many reasons some of these lands not taken back by registered documents. Though it is not a good way of doing things but this is the reality which in many instances leads to legal disputes.

if it is an ancestral property the gift may not be binding on son. Check with a good lawyer.

Just because the grandfather got it from his father need not make it ancestral. Facts posted suggest there had been a division of property by metes and bounds, and probably, execution of a partition or settlement deed.

I found that in case of ancestral property the person whoever looks after the property stands in the same footing as custodian or trustee. He manages the property for the benefit of the members or coparceners. He can not gift away a piece of property without consulting the members as the property is still belonging to all members as a unit until it is divided into shares. Even if he gifts by way of a registered gift deed it is not binding on the other members. He is gifting something which does not belong to him. The donee would not get any right in the property. At the maximum what the donee may get is  the portion/share of  property if it is split on the day of gifting.

Here is a case that went through all the tiers of judiciary from lower court to Supreme court confirming a similar situation.

https://indiankanoon.org/doc/1678680/

The facts of this and thee  judgment thereto, has no relevance to the facts posted.

According to my knowledge, The exact date of Gift deed, the date of knowledge of such gift deed by other co-sharers, the present status and enjoyment in Revenue records are relevant facts.