My brother took all the property of my dad fraudulently

My dad had Alzheimer's disease since 2015 and he died recently. But what I came to know is, my brother has taken all the property while my dad was in terminal condition. In that condition he was not able to identify anyone and he didn't even have the basic reflexes let alone signing a document where he gives all the property to my brother.I have a medical certificate claiming that my dad had Alzheimer's and a document which shows that all the property has been given to my brother.He used to assault my father at times and also didn't give him any treatment. My father used to live with my brother, his whole family abused my dad physically and mentally. What should I do now?