First of all, the question that can be paused is as to whether the property was acquired by your grand mother from her own income, or that your grand mother also derived the property from your grand father, or such other family source?
If, it was not a self acquired property of your grand father, then such property becomes the "joint property". There is a difference between the "joint property of the Hindu Undivided Family" and "Self acquired Property". Self acquired property can be willed away and given to the beneficiearies named, but the will for the "joint property" can not be made.
You will have to keep this distinction in mind. Now assuming for the time being that your grand mother acquired the property own her own and from her resources, without the contribution of his family members, or elder, then she has a legitimate right to make a will preferring the beneciaries. She may prefer to give to anybody, or she may set it to fire. It is her individual discretion.
Now, it appears that she has specified the share of each beneficiary, and therefore, one has to accept the will as it is, provided it is a self acquired property. The allegation that her signature was obtained by coercion etc.. is very difficult to prove in the Court. It is easy to make an allegation, but difficult to prove. A will can be made in any language. Making of will in english may not be fatal. It is admitted by you that your grand mother signed the same in english.
You may scrutinise the will and confirm as to whether the will is attested by two witnesses. If the will is in fact attested, the witnesses will definitely give the evidence in the Court that such will was prepared and executed in their presence and upon request, the will was attested by them.
Now, another facet of the case is that assume to the effect that it was not a self acquired property, but belonged to Hindu Undivided Family. In such case, your grand mother was not legally permitted to make out a will of the property that belonged to Hindu Undivided Family property. If it was a joint property at the time of her death, you have some case to challenge the will on the ground that the will was not permissible under the existing provisions of law, and therefore, the will should be set at naught. You ask the court for a relief to distribute the property amongst the legal heir and or the immediate decendents.
Hope, this resolves your issue in mind.