I (25f) have been living and going to college a couple hours from my family for the last few years and am moving back next week. My entire extended family is very conservative Christian and my relationship with my parents has been a bit shaky (but mostly normal) due to me living with the cousin that transitioned and went no contact, and then me being forced to come out as aroace. That, plus me living in a larger city, has showed me that i don't want to live in this state, let alone that tiny neighborhood, any longer than I have to. I want to fix up the house my parents helped me buy and then sell it back to them (that was part of our agreement when I eventually move away, as it's on the same lot as theirs) so I don't feel like I owe them anything when I leave.
The problem is my younger siblings. Making and keeping friends was always hard for me so I've relied a lot on their company. I've always had a great relationship with them and care a lot about their well being. The oldest (17f) is mostly fine and doing her own thing, and I'll be free to figure out our relationship as adults soon enough. The youngest (10m) is a great kid and I love hanging out with him. The biggest problem for me is the middle (15), who just came out to me as trans masc the other week. I've made it clear to him that I'll try to stick around as long as I can, and that he is more than welcome to live with me and our cousin when he's old enough to move out. So I've pretty much committed to living next door to my parents in a town/state I hate for the next 2 1/2 years.
I just don't know what the best way to help him would be. Our parents have been mostly fine at raising us, we probably wouldn't have had real issues in the relationship til we were all adults if it weren't for multiple people ending up queer. So there's absolutely no chance I could win any sort of court case or CPS investigation to get custody, even if I had the money to pursue that, and support them if I did win. I feel like the best outcome is for him to stay in the closet til we move, but that'll probably make him miserable. They already got suspicious and are cutting back on what he can wear, He doesn't have many irl friends cuz the kids in town suck and don't like his autistic vibes, most of his hobbies are digital/online, but they're cracking down on that cuz they don't like how much unrestricted access he has and will check his history (on the devices they know about lol). And I'm just worried that if I do too much to help and support him, they'll think I'm a bad influence and severely limit the amount of time i can spend alone with my siblings. And then when we do leave, I'll then have to worry about losing touch with the youngest, since he'll only be 13.
This is partially just a vent at this point, but I'd appreciate any advice I could get.
TL:DR How do I support my closeted trans sibling without pissing of our parents and getting him in trouble?