If you suspect your father is of unsound mind, has dementia or may not be competent to take care of his own property such that he would fall victim to someone who would abuse the relationship in such a way, you might wish to speak with your father about obtaining a power of attorney over his affairs and that way, any time she attempts to have him transfer something to her would be invalidated or impossible because you would have control. Consider this option as part of any testamentary documents he should be preparing (wills, trusts, medical directives, power of attorney).