Dispute between brother and Sister

Hi, We have a property that belonged to my late grandfather. My grandmother executed a will for this property, stating that both my father and his sister would receive a 50% share each. However, my grandmother only mentioned the property in the will, while other assets like fixed deposits money were already transferred to my aunt's account. Now, despite the will, my aunt is claiming her 50% share of the property as the same is mentioned in will. My questions are: 1. Can my father contest this will and prove its authenticity as false, or prove that the will was executed under pressure from his sister for executing the will of 50% share from my grandmother.? (My grandmother is expired) 2. What are the legal options available in this matter, considering my father is not financially stable but his sister is wealthy? We have all the documents available as proof of the money that my grandmother transferred to his sister's account without informing him. still she is asking for property share as per the will executed.