On great grand father demise property would devolve on his legal heirs 

Your grand mother can bequeath property by will 

it is not ancestral property 

property which has remained undivided for 4 generations would be ancestral property 

As it is an ancestral property she does not have the right to give it to anyone. She can only file partition so that her share may be decided. Challenge the will and file a partition suit immediately.

1. Will can be made only on self acquired property or one's share out of the ancestral property partition has taken place. 
2. As in this case, no partition has taken place, the Will in question is not valid and should be challenged before the high court. 

The law is, any legal heir to ancestral  property can bequeath property only to the extent of share to which he/she is entitled. Any transfer or execution of will more than the extent of share by a legal heirs invalid and not enforceable

Other legal heir can challenge the will it more than her share is bequeathed to anyone.

- As per law, an ancestral property cannot be transferred to anyone by way of Will .

- Hence, the said Will written by the grandmother having no legal value , as the property is undivided 

- All the legal heirs of grandfather is entitled to get the equal share in the property. 

As the property originally belonged to your deceased grandfather and since he is reported to have died intestate, the properties left behind him shall devolve equally on all his legal heirs. In the given situation, since your deceased grandmother is also one of the legal heirs, insofar as the Will of your deceased grandmother is concerned, it will be restricted only to her undivided her share in the property alone and not for the entire property. 

Therefore the beneficiaries of the Will can file a suit for partition to claim their share in the property which they are entitled to acquire by operating the Will. 

Since the Will is pertaining to the limits of Bombay, it is mandatory to get a probate of the Will through a court competent. 

Dear client I am sorry to hear that full stop in the case your grandmother cannot do that has a proper tenses will property and all the legal hair have equal right over the property and it cannot we will against one

Your grandmother legal heirs will get their respective shares as per Succession Act, class I heirs list.

And your grandmother's share can be WILL to her brother-in-law family members.

he can do will only for his share and not entire property without consent of other heirs

i need more information to answer better

first of all you need to ascertain whether the GGF purchased this property out of his money and then introduced that property in the common hotchpotch of the joint family OR did he consider that as his separate self acquired property

your GM i assume is claiming from her husband who is your GF who in turn would claim under the GGF

So the family tree is required to be studied

the GM can make a Will in respect of this property only restricted to the undivided share which she claims she inherited from her husband AND she cannot bequeath the entire property, prima facie

if a property which does not belong to the testator has been bequeathed in a Will then such bequest would lapse since there would be no property answering the description as a property which was owned by or belonged to the testator

if additional info is provided then a better answer with some specifics can be provided

for now your query is very limited and it requires some context