Newyearhere20, that sounds very complicated for you all, I’m sorry to hear that.
I think you should say something.
Firstly, because chest binders are harmful if worn wrongly or for overly long (and quite possibly harmful for any period of time, especially for a growing girl).
Secondly, I think you should get it out in the open so that she can talk to you and tell her how she feels and what she’s got on her mind.
I mean, you don’t know if she feels “trans” or if she’s just highly uncomfortable with changes happening to her body. If the latter, maybe you could offer to compromise and buy her minimiser bras? And period underwear instead of pads?
Maybe you could talk to her in the car, if possible, so you have a captive audience but don’t have the awkwardness of looking at each other?
If it turns out she thinks she might be trans, you might want to find out if she’s said anything to school. Some schools have been found to be facilitating “social transitions” without informing the parents.
It must be really hard being a teenage girl these days. It was hard enough for us, but now there’s constant social media (and porn) showing girls and boys the “ideal way” they need to look/behave/be.
Personally I never understood why films and books showed girls looking forward to starting their periods or saying, “I must I must improve my bust!” because those were the very last things I wanted when I was 13. I felt like my body was betraying me and that being a woman was rubbish and pathetic. I hated it when older women tried to be encouraging and talked about strong female energy etc 
Like almost every other teenage girl who felt this way, I got through it. I’m fine with being a woman and wouldn’t change anything. I gradually discovered feminism and that pathetic is the last thing women are. It takes time and experience to get some perspective. Womanhood can’t really be escaped and along with the inevitable changes comes the discovery that there’s lots of ways to “be” a woman - you can’t “do it wrong” - you just are one.
Check out the websites I mentioned in my last post. I’m sure they have some good advice on how to help her come to terms with puberty and the uncomfortable physical and social changes it brings, without making things too confrontational or putting her on the defensive.