Can my father contest the will under duress and pressure?

We own a property that belonged to my late grandfather. My grandmother executed a will stating that both my father and his sister would receive a 50% share each. However, the will only mentions the property, while other assets, such as fixed deposits, were transferred to my aunt's account without my father's knowledge. Now, despite the will, my aunt is claiming her 50% share of the property. I would like to know if my father can contest the validity of the will, asserting that it was executed under duress or pressure from his sister. Additionally, I seek guidance on the legal options available to my father, given his financial instability compared to his sister’s wealth. We have documentation evidencing the funds transferred to my aunt’s account without my father's consent, yet she continues to demand her share of the property as outlined in the will. Thank you for your assistance.