My 83 year old dad is easily coerced by everyone. My brother just came over and "bought" my dad's truck. Any advice?

His truck was his prized possession. I too am happy that the truck is staying within the family however it does not stop there. My brother used my dad's condition to get the truck way cheaper than what it's worth. My other brother came and borrowed a large amount of money from him knowing my dad will forget and that's the only time he ever comes around. I have a sister whom is his POA and the only thing she does is take him to Dr appts, that's it!! She doesn't even provide financial help to cover my dad's expenses, we cover them all. I don't know how much more I can take!! I have learned a lot about his condition as I've been here in my childhood home caring for him and my mom for 14 years now. My mom just passed 2 years ago. Now it's us and my fiance. He and I are my fathers only caregivers. So out of frustration from seeing me hurt my fiance will often try to discuss with him how the others are doing him. It is useless but my fiance is so tired of watching me stress, get hurt and aggravated plus my health has been an issue (recently lost my right kidney to cancer and now there are spots on my left kidney, liver and back plus I've been told I'm in early stages of COPD and I've never smoked at all). My fiance has stepped up and taken care of my dad on a level that his own children can not understand or appreciate. My fiance is simply to the point of aggravation with the entire situation and it's affecting us but it's affecting my dad as well. Does anyone have any suggestions? And I feel as though we shouldn't discuss their lack of concern with my dad because that will only continue to make things worse? Any advice? I love my fiance and I do not want to see this issue causing us to break up.