Son's control over parental business & property

A very aged father of 3 sons owns his business which he never ran by himself. Qualified in Business and skilled too I have immense contributions in turning around the business over a decade. Due to my close involvement in my profession elsewhere, in the last few years another son of medical background has prevailed on my father by mental pressure / threat and managed to bag the financial control (purchase) defying others. Currently taking advantage of old age /indecision/ill health of the parents he dumped his personal belongings into the Business premises and is getting several authorization letters signed by him in his favor without consulting other brothers. Over the last few years father becomes of unsound mind losing memory and prudence by the day. Despite that the eldest son has failed to run the business to break even, the son wants to prove to the public authorities, his full control over business and property. He is not amenable to legal mediation and refuses to discuss for settlement. My father is indecisive and servile before the intimidation by his eldest son , who meets him in our absence. Another son not from non-business background, too is highly disappointed of the situation. Please advise What action needs to be initiated against Whom immediately to protect my Rights over business and property being a skilled and qualified business Manager serving elsewhere.