You're not being difficult. You're stuck in a difficult situation. Big difference. (And wow! Oldest is a girl? I wonder what that's like. My entire family -- siblings and cousins -- always start with boy, boy, girl, optional. lol)
And your oma (love that name) probably is poor enough to not be able to help. A single woman is going to have a tough time bringing in children so young. (Not you. Your younger sibling.) There is the possibility she might be able to get paid as a foster parent, but I don't know that side of Family Services to know for sure. All I know is the government will pay an older sibling some money each month if they take on raising their younger siblings. How much and how is also outside my knowledge.
As for your pastor? Well, you've made it this far and you're good for a while longer, right? (As in you're not in imminent danger and she's at least talking like you'll be staying in that school for the year.)
If you're not good, (and I get it's possible you're not since you don't want to tell too much because folks will freak), it really is time to call Family Services. I know you don't want that because of what happened last time, but if it's a choice between that and being in physical danger, choose that anyway. It does royally stink, but it too is temporary. Physical danger is deadly, even if it hasn't happened yet.
If you are good, then wait for your pastor to return. I am assuming he comes back by September. And when he returns, start with your fears first. Tell him why you're scared to say too much and feel him out. I worked in a Runaway/Throwaway Shelter for teens. (It was called a runaway shelter, but most of the kids were thrown out.) Every single one of them who lived in bad situations had a new place to live -- outside of foster care -- within a week. We only could let them stay there for three nights, so many got a safer place within that three days. What usually happened was they went with another family member. Sometimes it was a friend's parents. Sometimes pastors did help. They usually knew where they wanted to live, but were sure the parents wouldn't let them and family services would put them in an orphanage (hey, the 1970's. It wasn't reality then either, but they had just gone out of style and the kids didn't know. lol) or foster care. Truth is not much foster care to use, so anyone willing to take the kids was fine while family services investigated. I don't remember a single teen who went back home. I do know about a third of them didn't last where they were, but usually because it wasn't permanent placement so the kids bolted again.
That's where your pastor comes in. Family Services probably already know him. If not, they lean toward trusting pastors. If he can find y'all places to live, you'd need him to make it permanent placement. And once there, then they investigate. So, you're safe as long as the investigation goes. Added bonus. She's not family. She's not blood, so she probably won't fight it. And, if she does fight it? Again. Not family, not blood, so she'd have to prove herself.
Or, if the pastor knows lawyers that can help. (I don't know what the congregation is made up of -- job wise.) More legal wrangling before they even get to the governmental wranglings.
I'm guessing stepmom is also Mom of some of your younger siblings. That might make it impossible to pull everyone out, but if they investigate you, they investigate all. Judging from her past decisions, (married your dad and how she treats you), she cannot pass mustard for you. They'd be hard pressed to let her keep her kids. If your pastor has a better plan, the odds go up she doesn't. The odds. She might actually like her own kids and treats them better.
And yeah, that probably does mean you don't spend time with your siblings growing up. Temporary circumstances. You're stuck with the reality of no legal say as kids, but all bets are off once each child becomes 18. I've past 18 years three times so far. I've got more in me. There is more adult time than time we have as kids. No choice means you will meet again and become family. (If you had major fights with your siblings and you weren't talking to them, then less likely to happen, but that's not happening.)
I know someone who grew up with all the abuse one can get as a kid. His mother used to threaten him and his brother with, "I can always drop you off at an orphanage." He laughs now. "If I had known any better, I would have packed my own bags."
What you're going through is bad. Really bad. What you fear is better. Talk to your pastor when he returns. Start by testing him without telling much of the story.
Going back with my dad after Mom died was my version of terror. The only reason I did was to take care of my little brother. I was given a choice before Mom died. A pastor told me my brother and I could be hidden within his church. (They had a retreat out in the middle of nowhere, and he'd said we could stay there.) He never found out the whole story, but that alone gave me the confidence to make my own decision. It is doable without opening the whole bag. I never told him why Dad scared me.
.....