My brother's birthday is coming up soon and all my family (including me) are gonna go to his house and have dinner.

I'm kinda freaking out.
Firstly, he has a girlfriend. Did he say he has a little sister? Was he cool enough to say little brother, if so, I don't think he knows the name I chose. Or did he tell her I'm trans? Did he just pretend I don't exist? I have no idea.

Secondly, my (other) older brother still uses female pronouns. At first he was cool and used male prounouns and stuff but now he just uses female pronouns. Stopped using my birthname though, at least. But it will be awkward to meet someone for the first time and introduce yourself as Thomas and then someone goes and says "she" all the time about you.

Thridly, my grandmother has a new partner too. I've met him twice, and yes, both times were extremely awkward. Does he know my preferred name? I DUNNO.

And last but not least, will my brother's girlfriend bring any relatives? If so, what has she told them? That he has a little sister?

I dunno what to do. Right now, I'm pretty sure I'll just lay under my bed and hope my nobody finds me and then stay there till the end of December. That sounds like a perfect plan.

But anyway, anybody got any advice on what to do about all of this?

This depends on your confidence level. If you're concerned about it and think the family might be receptive it might be worth telling them that you would prefer he/chosen name for this event.

I say this because that is what I wish I would have done. I went to a family reunion 1 three months after coming out to my immediate family, 1 week after starting T and 2 weeks before my legal name change was processed. At my mom's request I agreed to pass as female with the family as long as I could complete the trip as me on the way there and back. It was the longest three days of my life. I hated having to introduce myself using the wrong name and by the end of the first night people were asking mom about me rather than just coming to me. By the end of it I was ready to stand on a table and shout "I am (chosen name), I'm transgender and I need to be accepted as the man I am". Due to quick corralling by mom & grandma I didn't get the chance and we started on the drive home.

That day marked the last I would ever introduce myself by my given name because I realized I lack the ability to lie about who I am.

I don't know if that helps you any. Be sure to take care of yourself. I survived the reunion because of a friend who called every night to wish me goodnight, using my real name to help me get through it. If you're ready to start correcting people this may be an opportune time, but if you're not be sure to plan how you'll handle people doing things that hurt you when you can't/won't make them stop.

If I were you, if you can, speak to your brother and ask him if he has said anything to his girlfriend and what. Maybe he has explained it to her and shes fine with it all, that could possibly work out best if you present as male and your other brother uses the wrong pronoun, atleast she will understand what is going on.
And/or you could speak to your other brother too and remind him of everything and politely tell him you need him to go back to male pronouns.

I have been in similar situations. One was when I was 15, not on T, out to my immediate family, but not able to live as myself or introduce myself properly. I still dressed like a guy though, so I passed to people who didnt know me/didnt hear my immediate family using the wrong pronouns about me
We went to my Aunts birthday party, just to say hi and whatnot. There was a distant cousin Id never met before, he was probably in his late 20s. Basically, we all were chatting, I could tell he completely read me as male, and because I just said hi, but wasnt in the conversation,nothing came up to "out" me to him.
About a year ago, (Im completely stealth and living as male by now) Im out at a bar, and I bump into the distant cousin and he recognizes me and starts being really friendly and acting like weve been mates for ages. He seemed a cool guy, but also like he would not have understood/been friendly had he first viewed me as female and then seen me all masculinized, I could just tell he is like that.

Anyway, although my family didnt actually help by using my chosen name and pronouns, by not using the wrong ones, I was able to present fully as male which saved me trouble and awkwardness 4 years later!
So, I think it is best to try to get your family to help you out here. Speak to them.

Good luck.

Thanks for the replies they were great.
I'm gonna try and just man up and talk to my family before and tell them I would prefer male pronouns or at least no female ones, and to be called by my name.