Regarding Validity of Will

Dear Mr. Sir, Kindly advise on the following: We bought a flat in joint name of mine and my father. We executed the registered will when my father was perfectly well around 3 years back. Now he is suffering from Dementia (brain disease) and cannot sign or understand anything, since 1 year. He is staying with me. According to his will the property goes fully to me after his demise, since my brother already has one flat solely in his name. I need your advice on: 1. Can my brother change the will of my father by his fake signature, since he is being doing it on other govt. documents where my father cannot sign? Is that has full worth, he can easily get two witness who are in his favour? 2. Will this new fake "will" be a valid "will" since it cannot be registered now as my father cannot do anything and is mentally ill? 3. What should I do to protect the rights of my children since I have this house as only asset? 4. In case my brother goes fair and no new "Will" is executed (fake will) then regarding the registered will, I have another question that my brother, is keeping the original registered will with him, Can a duplicate will from the registrar office has same value as the original in case he does not give it to me and I need it to sell the property after demise of my father? 5. In case I want to sell the property can I just use the duplicate copy of the will or I have to get the consent signature of my brother also after demise of my father? Thank you.