What to do if my stepbrother/sister are sabotaging efforts to sell the house to pay for care for dad/stepmom with dementia and I suspect greed is a motivator?

I'm dealing with a very difficult family issue in trying to provide care for my dad. My dad has been remarried for 40+ years to a woman named Jutta who has 2 children Steven & Debbie. I have always struggled to get along with them because they have a very toxic dysfunctional dynamic which scared me off. No family is perfect, certainly not my own either. But this was really difficult to be around. Debbie struggles w/ a constant cycle of addiction and rehab, and pathological lying. Steven is a malignant narcissist bully with alcohol, marijuana and gambling issues. This is not an exaggeration... people who know him agree. They can be very nasty, vindictive and manipulative. So, I've always steered clear of them as much as possible and spent time with my dad separately.

In Sept. my dad had his first stroke because he refused to take his medications. This caused a mild slur, right side weakness, and the beginnings of Vascular Dementia. Debbie was coming out on weekends long before the stroke to help them take care of their house since they weren't able to any more. No one told me this. My dad & Jutta moved 3 hrs away during Covid, so I didn't see my dad as often. I appreciate that Debbie did do a lot for them, but she also spent an enormous amount of time at the casino gambling. The house was truly horrifically filthy to the point of being a health hazard.... so I'm not sure how much help was really happening, but I still appreciate what she did. Steve never helps with anything, ever, but would come visit so he could gamble. After the stroke, I strongly advocated for them to move into a senior community because I saw that Jutta had full blown dementia, which I didn't know. Dad always said she doing fine. Debbie originally agreed it was needed, but then later denied it to Steven. Steven chewed me out w/ nasty phone calls and many text messages informing me that I wasn't a part of their family, they would take care of them by coming on weekends and I wasn't welcome there. I warned him that my dad would stop taking his meds again, and then he would have a bigger stroke, or die. If he didn't die, he would be so impacted that he would have to go into full Assisted Living and not an independent senior living community, and then we'd have a gun to our heads w/ no time to plan or adjust to the change. They would not listen. My dad would not even discuss it, and got very angry. I'm ashamed to say I gave up, but I had no other choice. My dad is VERY stubborn, and Steve & Debbie were egging him on.

I was right. My dad had a bigger second stroke six months later, which is this month. He stopped taking his meds again, and would lie to me about it every time I called. Jutta stopped taking hers meds too, and her dementia progressed. My dad now has dysphasia (scrambled words), is very weak, and now has significant Vascular Dementia. They absolutely cannot live on their own, and the doctors all recommend Assisted Living. Sadly, my dad opted not to sign up for LTC insurance (his job offered it), has very little savings and a fixed income... not anywhere near enough to hire a Home Care Aide so they can live at home.

So now the only option to pay for their care is to sell their house, which they own outright, thankfully. I have Medical POA for my dad, and Steven's wife Marianne has Financial POA. I was asked to do financial too, but didn't want to deal with Steven & Debbie. Given both their history, nobody trusted them w/ anything financial. Marianne is a good, stable person who has their best interests at heart, so it made sense. Steven &Debbie are fighting us at every turn about selling the house, even tho it's the only option. They're very verbally abusive to Marianne and making an overwhelming, bad situation a hundred times worse. Meanwhile they have done zero to help with Jutta & Dad, or even visited him! I think it's about greed. They want to inherit the house. Marianne keeps caving in to keep the peace. I'd be grateful for any advice!