Your stepparents are your father's ex and her current husband, right?
You aren't growing up with a normal childhood. And the people raising you aren't normal parents. I don't really know why your father is where he is, (not my place to get into the details here, but you know what I mean), but I'm assuming some of what he did was during the time his ex was living with him, right?
People live with their own kind. Both my parents came from families where education was highly valued, and family was the cause for all decisions made. (As in can't move across country because my family might need me. Can't go to far away on vacation because my family might need me. Won't take the promotion because that means moving and...)
And hubby's family married their own kind too. They both valued having enough money to enjoy their passions and their status above minor little things like worrying about if the kids had clothes or toys. Neither thought education meant much, so they didn't care if the kids went to school or did well, until the report cards showed up, at which time they were all punished for getting bad grades -- no matter what the grades were.
In like kind, I suspect your dad's ex married a bad boy, because that's what she was raised to believe was a good choice.
Add to that, what you've gone through truly strained the finances, so they were ultimately stressed in the two biggest stressors adults can have -- very sick child and finances. It doesn't get worse than that for any adult. And you know my story, and I'm saying it doesn't get worse than that. THAT tough.
Meanwhile other kids came along. And through all that they had to survive, take care of you, try and raise the kids, bring in enough money to take care of everyone, and honestly? It cannot be done smoothly. Something fell apart in all that. Often, considering we're talking you're living with your stepmother and stepfather. (Most kids get one or the other, not both.)
In that falling apart, all you kids were different ages. I see no way around what the adults in your family went through other then drop the ball on the kids least needing to take care of the one who needed the most -- you. It's what most parents would do. And the reaction to that differs from child to child. You probably ended up with more attention than your younger siblings just because your needs were that great that you truly, honestly needed the attention. (And I'm sure you were still scared out of your mind. This was a series of problems that might be fixable -- and thankfully were -- but no one counted the guaranteed fix, so you were all stuck together for better or worse.) Still, I bet you have resentments because they dropped the ball more than once. They didn't see something. A sibling was sick when you had something huge going on. Or even whatever happened with your dad.
Meanwhile, the 12-year-old brother is watching his parents go away to see you. The 10-year-old is watching the 12-year-old freak out and no parents to save the situation. And the younger ones see more and more of this. Worse yet. None of you were as old as you are now when all this hit the fan, so you really couldn't express any of this in definitive ways.
Now your parents have to figure out how to put this family back together again. It's tougher on them than anyone will get, because they did see what was going on most of the time, but didn't have enough to handle it all.
Add to that, in any given family, the oldest does get picked on as the one who will continue rising up to meet the American Dream. (You, my oldest brother, and every oldest child I know are the ones to show the rest of the family how to get better. That family legacy stuff really stinks like a sweaty armpit for the oldest child.) The oldest also gets the brunt of punishments specifically because they're the only kid who doesn't get to watch an older sibling push the parents to learn how far to push them to get our own way. (Hey, I learned not to cross the street without Mom there because I saw what happened to oldest brother when he did. And oldest brother resented me, because I'm Number Three, and we all pushed enough at the same time so I got to cross the street by myself the same day oldest brother did. Every time we got less rules, it started with me and the thought that, "well, if we're letting her, might as well let them too." Of course older brothers resented this. lol)
So, I get you're in the toughest spot, but you have to keep remembering, your stepparents have never been to the point of being pretty sure all the kids are healthy enough to start making plans again. And, no matter how many siblings there are, they really have had little training on how to raise kids in this situation.
Keep remembering something. No matter what, they do love you. I've met plenty of kids who were turned away because they weren't biological kids. I've met kids in the foster system, who know they have siblings, but haven't seen them in years. And I've met kids that were given up because the medical care needed was more than the family could support. As messed up as your stepparents might be, they are still keeping you kids together.
Your situation is messed up in more ways than people can comprehend. BUT you have two adults willing to try so hard that they are getting counseling for their kids because they know why it went this south. AND they're listening to the advice given. They may never get it right. I don't know what right looks like for your family. BUT they truly, truly love you all.
That beats most families I came to know through my time as a drug rehab patient and counselor, a runaway/throwaway shelter counselor, and a group home parent.
You are loved!