Is it permissible for us to try to obtain a medical certificate stating that our father is unable to sign or control his hands due to Parkinson's disease, for fear that he might transfer ownership of his house to his wife, who controls him and constantly demands money from him?

The father's actions, such as his marriage and living away from his children, or their fear that he might register his house in the name of his second wife, do not warrant placing him under interdiction. It is obligatory for the children to honor their father and treat him with kindness, and to provide for his sustenance and medical treatment if he is in need, because supporting parents is obligatory when they are in need. The father does not have the right to bequeath any part of his inheritance to his wife, because that is tantamount to a will for an heir, which is not valid unless the rest of the mature, adult heirs waive their share.