How can I convince my dad to protest his assets from his girlfriend?

My dad is 63 and has had cancer for 5 years, and has been dating his girlfriend for about the same amount of time. It had been my assumption that he never intended to marry her. A year ago, he had a medical procedure done which impaired his cognitive functioning for a short time following the surgery. During this time, he announced that he was engaged to the girlfriend because she was badgering him that she wanted to get married. My family and I made it clear that we disapproved of her motives. My dad and his girlfriend live together in a house that they each own half of, with this woman's two adult children. She has significantly fewer assets than he does, and my family and I believe that this woman is after his finances. A few months later, he called off the engagement. They have been going through rough times in their relationship. She is mean and rude to my siblings and I, and they argue a lot. I believe that he is only staying with her because he does not want to be alone or live alone while he is ill. He just announced that the engagement is back on, and the wedding date is 6 months from now. I believe that he does not have any diminished mental capacity, but that she is saying that she will leave him unless they get married, and he is afraid to not be in a relationship. My siblings and I have made it clear that we think she is after his money, but he says that he "loves her" and wants to make her happy. How can I convince my dad to protect his assets from this evil woman?