First, you already know that engine is done. It's not even a viable core at this point, IMHO (I've got some Toyota experience). If I were in your shoes I'd say, "Hey sis, I love you and tried to help you out. You didn't listen to my warnings about regular maintenance and generally treated my car very poorly. Your options are to go buy you own car now, or drive this one until it absolutely will not run. Might be tomorrow, might be a year from now. Either way, you'll be walking and paying a tow truck to bring it to me. Then I'll have to clean up your mess." Spoiler alert; you'll be paying for the tow truck.
She'll likely opt to drive it until it seizes, you know, because she's busy. Doesn't matter -- engine's already ruined.
I can't answer what to do with it. The car has the same value running with no oil pressure or with the engine seized. I can understand some sentimental attachment and the idea of not wanting to give away all the new parts and labor. (You may remember my rant about my daughter not wanting to drive the Neon I basically restored for her -- I absolutely get it). If it was just the engine, I'd fix it. But if the cosmetics are that bad, maybe that just doesn't make sense. You have 4 other cars, so it's not like you need to fix it. Might be best to cut your losses. I can't say. That's a decision only you can make.
Sounds like you kind of expected this from her. I know it stings but it's best to just suck it up and write this good deed off. You just got proof that no good deed goes unpunished. She'll always be family and there's dozens of holidays, birthdays, weddings and funerals ahead. Don't ever bring this up again, and NEVER GO OUT ON A LIMB FOR HER AGAIN. Sometimes being the grownup is frustrating and expensive.
Sorry your sister sucks.
You pretty much nailed it. I couldn't have summed it up any better myself. I have a lot of issues with my sister, extending far beyond her just beating up on the car. We just have very opposite personalities. She'll go out of her way to do completely unnecessary favors for her friends, but when it comes to her own family, she wouldn't piss on us if we were on fire.
We are civil with each other but it's nearly impossible to have any type of real conversation with her. Basically limited to talking about the weather. She lives in the same house and didn't know that I had gotten an associates degree and had already gotten a job and been working there for 3 months. Anyways I'll chalk this one up as experience.
As for what to do with the car once it does leave her stranded, it hard to say. As you said, I have 4 other cars, I don't exactly need a fifth. It doesn't have a straight panel on it, just about every panel has those deep scratches in it, the clear is starting to peel on the C pillars, and it would need front and rear bumper covers. The interior would need a new drivers seat, a headliner, a steering wheel and I wouldn't even waste time trying to clean the carpet. I could probably live with the exterior being ugly, but the interior is just ****ing disgusting, I couldn't stand to sit in it. It would be a lot of work but keep in mind you are talking to the guy who restored a rusted out $300 truck because I liked it and the A/C still worked.