Today he chose her. He's moving there in January, and it feels like I am dying. I have waited years for him to come home, for him to spend time with my sister, her son, our mother etc... and then he chooses some chick he's dated for a little under five months.
Sorry, but, well, duh. Yes. Of course he chose her. And you know what? That's totally normal. In fact, I'd be a little weirded out if he didn't "choose" his serious girlfriend over his sister at this point. How would you feel if your brother was upset that you started living with a boyfriend you were really in love with? Would you leave the love of your life so your brother wouldn't be upset?
Look, I am a twin your exact age. I've been living with my sister literally our whole lives. She is my best friend. And if she met someone wonderful and they loved each other enough to go away together and start their lives, yes, I would miss her terribly...but I'd also be super happy for her that she found someone! This is part of growing up and it's normal.
I've also been the girlfriend in this situation with a jealous sister who resents my very existence for reasons I still don't know after 2.5 years. My boyfriend's sister was really upset when I started dating her brother like you are right now. She hadn't even met me, she didn't even know me, but I could tell she really didn't like me. It didn't matter who I was, she really hated that her little brother had a girlfriend. I went OUT OF MY WAY to be nice to her when I visited, even though she acted extremely cold towards me. It was horrible. I still get anxiety when I know I'm going to see her because she makes me feel so alienated. The fact is, she was jealous of the attention I got from my boyfriend. Which I failed to understand because I am a completely different person with a completely different role in his life. Yes, it's normal for a couple to hold hands when walking down the street after dinner. (She would say it was gross).
I love my boyfriend and he loves me. We've been together for 2.5 years, but at 3 months, we knew we wanted to get married. And while I get that you and your brother are close, honestly, I don't understand the anger.
I'll tell you the real talk her family gave her. Honestly, what do you want? Do you want your brother to be single for the rest of his life and spend his life celibate and just living with you? Because that's what it sounds like you want. Five months is a pretty long time to be in a relationship and it's certainly appropriate at that time to be serious. You make it sound like she's some random he's been shacking up with, when it sounds like they've been in a loving relationship for a while, with no sign of stopping.
This isn't about you at all. This is all a normal part of growing up. I don't think you're aware, but your attitude is going to come across as selfish to your brother, who probably just wants you to be happy for him. I'm sure he'd be happy for you if you met someone wonderful that you'd be willing to move away to be with. You just accept it. It's literally none of your business.
And please be kind to your brother's girlfriend. Her only fault is loving your brother and wanting him to be happy.