I'll preface this answer stating that I am queer, but have not come out to my extended family (what's left of them), aside from having my Facebook name match my new name and being friends with my cousins there. Despite being vocal in the past about my identity, I don't know what all my family has seen or if they even call me Lux. This answer will be generalized, but I still think it will be applicable (and can help with other social circles).
Before coming out
First and foremost, ask your child what she wants. Coming out is still a very personal issue and only the individual gets to dictate how they come out. If she wants to make the announcement herself then your job is to be present and to stand with her. However, it's possible that she'll want you to announce it. You did say you were having a boy when pregnant with her, I think it's still appropriate to announce that you actually had a girl. (Usually outing someone is a big no-no, but here it's okay imo)
Either way, the important thing to do beforehand is to get on the same page as your daughter. Ask her what is okay to say and what's not. Do you tell family only her new name/pronouns or do you give the whole story of her journey so far?
Then ask how she wants to come out. Does she want it done in person, over the phone, or even via cheesy family newsletter? She probably has been thinking it'd be in person (I've been assuming it would be), but there are other methods each with its own pros and cons.
Then you should warn her that she may get intrusive and inappropriate questions. Within 5 minutes of me coming out on FB I was asked if I was getting the surgery. It seems when it comes to trans issues all of the mores and taboos get thrown out the window. People don't normally ask each other about genitalia, but now that your kid is trans she may get asked questions. Tackling these questions can be as simple as saying "I don't know" or "We're still figuring out where this gender journey will take us" and remind family that this is still a pretty private affair. On the flip side, you and your daughter can be as open as you want. Announce that she'll do hormone replacement therapy (HRT) but no surgeries at the moment, or voice training and make-up lessons. Ultimately it's up to your daughter.
No matter what method of coming out is used, keep things light and state things matter-of-factly. We're still in a society where your daughter's very existence is controversial and some family members may not be supportive. I believe just telling it how it is with confidence can go a long way. I have a cousin who doesn't know why my name changed and pressed my mom for information. I assume he thinks that I am trans and I also assume that he's not cool with it. However, she just held firm and said that I am an adult and get to make my own decisions. It shut him down. You can give a similar argument since your daughter is a young adult now and making a name for herself (literally in this case, I guess).
After coming out
After the announcement is made is when it becomes your time to shine. When you're an ally, the name of the game is solidarity. When family and friends misgender/deadname your task is to correct them. More often than not it's just a slip up (it is a 17 year old habit calling her by her dead name after all). There are a couple of different ways to correct, the standard being repeating the sentence with the correct pronoun:
Family Member: When he and I saw the latest blockbuster movie...
You: When you and she saw the movie...
One that my friends have used was the ouch/sorry method:
Family Member: When he and I...
Family Member: Sorry, when she and I...
Personally I'm not a fan, but some people like it.
On the subject of apologies for misgendering/deadnaming, when people slip up they should apologize, but only once. People can overcompensate with their apologies:
Person: When he... Oops, I mean she. I'm sorry.
Trans Person: It's okay.
Person: No, I'm really sorry. I swear I'm such a good ally. I am so accepting of trans people (etc)
It's unnecessary because mistakes happen, but then it also highlights that the misgendering happened and then puts the person in the spotlight as they try to [humble]brag about their being an ally. You can help shape how people interact with your daughter and how they talk about your daughter simply by reminding them the correct name and pronouns and even teaching proper trans etiquette. Based on comments, it sounds like the little sister is doing a pretty good job of this already.
In general, hop online and see what trans people have to say. Reddit is a potential source, recommended by Pyritie in the comments. Getting educated is a great thing for an ally to do and taking the time to Google and read is the best step. See what trans people have complained about when dealing with cis people and try your best not to do those things and correct people when they do those things. Continually ask your daughter when it's okay to confront people when a situation pops up and you weren't sure what to do.
Regarding the girlfriend
If your daughter's sexuality is also going to be part of the discussion ask your daughter and her girlfriend what labels you should use. You say that your daughter is bi, but her girlfriend may not identify as such. Maybe pansexual is more appropriate, maybe lesbian since this is a women-loving-women (wlw) relationship now. The point is, figure out what everyone is comfortable with then follow the above steps.
Good job on being a supportive mom!