my dad's (68) wife (60f) is trying to ruin our relationship

I'm only posting here for unbiased advice as I don't know what to do.
I am in my thirties (31 f) and I have a younger sister in her twenties (28 f) who is chronically ill. My dad (late 60s m) got married to a woman named Tina (60 f) a few years ago, relatively soon after they started dating. She was never rude to me, but she wasn't ever warm or active in getting to know us either.
After they got married, my sister and I started noticing that he would prioritize her family over us. Small things, but they still bothered us. I tried to bring it up, but it always got dismissed.
A couple years ago, they moved far away to a cute house. I was sad that they were so far, as I can't afford the gas to go see them all the time, but happy that they found a house they liked.
Recently, my father was diagnosed with a heart condition. Last week, he was in the hospital and I was not contacted until I asked where he was. The reason I knew he was in the hospital was because Tina decided to send my sister a series of extremely venomous text messages blaming her for my dad's health. Her reasoning being that my sister hasn't been able to afford the car payment my dad co-signed on. My sister has been barely scraping by for years. She does not have a lot of money. She also has some very bad health conditions that are exasperated by stress. In that text, she accused my sister and I of not caring about our father because we can't afford to come visit all the time.
My father got out of the hospital and is supposed to be taking it easy. Yesterday, I got a phone call from my dad talking about my sister's financial situation and I hear Tina screaming in the background. She is saying things like she's done with my sister and doesn't even want to be in the same county as her. In the aforementioned text message, she said she would not let my sister see my dad if she came to visit. This conversation really shook me so I asked my dad if I could call back later to have a conversation between just me and him. He agreed.
My dad and I later had a conversation where I heard him out, he was convinced of a few things about my sister that weren't true, and then I heard Tina in the background so I told him that I loved him but I'd have to hang up as he didn't respect our agreement. I was calm, as was he during that conversation. During that time, I learned that my sister being unable to pay had ruined his credit, which I agree is fair to be upset over, but I have an issue with the venom Tina is now spewing at both of us.
This morning, I woke up to a text from my dad (I knew it was Tina), asking if I thought they should get separated. I responded that I didn't know and he knew his relationship better than me but I want him to get happy.
Tina texted my sister last night saying they're going to move to a different state, so in my response to the text from my "dad" aka Tina, I said that I thought moving further away was a bad idea and I wanted us to live closer as a family.
An hour later, Tina texts me thinking she "got" me, saying that text was actually from her and they're never going to separate and she's going to cut off all contact with me and my sister.
I have no idea what to do. This is stressing my dad out and I'm trying to be diplomatic, but Tina keeps making everything worse. Please leave your advice.
Tl;Dr
My dad's wife keeps instigating issues with us
Edit:
I'm seeing this come up so I'll address it here. My dad is not financially supporting my sister at all. He co-signed on a car loan for her years ago. She is not spending money frivolously, she doesn't have much money at all. Sometimes she can't afford food because of bills. I am going to help her pay off the loan so this nightmare is over.


Source.

Let your dad be happy, he owes you nothing. we cannot make someone love us, nor should we try to guilt them into something. why should a parent "support" a healthy adult child.
the more you interfere, the more alienation will occur. tina doesn't sound like a very nice person. but that is your dad's choice. as they say, he made his own bed, let it lie in it.