It doesn't sound like it fits perfectly in black and white to me. I don't thinks she was right, at all. Since you were presenting as your true female self, whether or not she was trying to avoid discussing the subject with her friends, she's going to have to. She introduced you, while presenting yourself as your true self, as her "brother." Which will raise questions. The friend might not have asked those at the time to be polite, but I'm pretty sure that she has questions now and will ask your sister about the gap in how you were presenting and how you were introduced.
So sitting down and having a thorough and calm discussion with your sister is still my top suggestion.
Like I said though, it don't think it is so black and white. While she's known for a year and had time to come to terms with it, it's only been a month since you started going full time, which is another stepping stone for both you and those around you. She may have been content in believing she could hold off on what she may see as an awkward discussion with her friends, especially if a friend or two turn out to be less than accepting, she might fear having to cut ties with them to stand by you. That might have been something she feared and was putting off.
And now you're full time, and she has to take some time to adjust to that and come to terms with the fact her previous mindset of "I don't need to tell my friends yet" no longer applies. I think you should be lenient on her for now, and let her adjust.
You were not in the wrong at all. And She wasn't right. But I wouldn't go so far as saying she was wrong either. She might be hesitant to face an awkward situation that could result in her having to cut ties with some of her friends, so be lenient with her.
As for bring it up, start with expressing that you do understand where she might be coming from. That it you realize it is not an easy situation for her either, and that you realize how scary it can be to explain it to her friends. Especially if they might not be as accepting of you as they should be. Be calm, and considerate, and understanding of her side of things. This will stop her from feeling the need to defend herself or feeling offended. If you make a point of showing her you are considering her side in this, and don't see it or her as being deliberately hurtful in any way, will go a long way.
But be firm too. Explain that it is hurtful, and how much it hurts you, and that you aren't her "brother" you are her sister. Explain that it's not a phase, and this is happening, and whether she likes it or not, she's going to have to explain things to her friends. After all, why someone introduced as her "brother" is appearing and dressing female is going to need to be addressed at some point to her friends. She's not avoiding that by calling you her "brother" and that by showing her friends that she is accepting and sees nothing wrong with her sister, by introducing you as such, would actually make that inevitable conversation with her friends easier. They will pick up on your sister's attitude and acceptance and go with that. Where as, introducing you as her "brother" and then having that conversation brings up further questions of why then she introduced you as her "brother", as opposed to her sister, and prolong the conversation as they try to understand what is what.
Explain to her that she is your sister, and you love her and she is important to you. That you don't want anything getting in the way of the relationship you to have, and you hope she can understand your feelings and why this is so important to you and how much it would mean to you to know she's in your corner and accepts you for you in all situations.
And good luck. I hope it all works out with you and your sister.