Oh wow, I've read through this entire thread and really feel for you. I always hated being an only child but maybe that wasn't so bad after all (based on not just your own, but others experiences too). The way she's been acting towards you is awful, although I haven't grown up with kids I work around slightly younger than that age group, I can understand at home maybe they act more immature but I'd be genuinely surprised if I witnessed any of the 10-11 year olds, or even a bit younger, acting that way I'd be pretty shocked. She sounds extremely immature and please bear in mind that I'm not a dr but I'd be inclined to think she has mental health issues of her own. However that in no way cancels out your own issues and the only advice I can think to offer as someone who doesn't know your situation fully, is to tell her you're doing your best to get along with her and help her enjoy the game, but her behaviour isn't okay and if she carries on being mean you won't connect with her and help her any more, full stop. (If necessary I'd even consider asking your parents if you can have a bolt on your bedroom door providing you unlock it at their will, considering you sound mature and may allow you to relax and play the game on your own, or just have your own time away from her!) Possibly could try something like "if you're not nasty to me for x amount of time, I'll let you visit Celeste on my island" but at the same time she doesn't sound all that reasonable, and as a 14 year old yourself it's not your job to teach her to be reasonable.
I hope any of this helped in some small way and I wish you every bit of luck with the situation you're in <3 You sound like a great sibling, considering!!