Can a platoon sgt tell a female soldier that she is required to wear a bra when in civilians?

With 21+ years of military experience. It is subjective at best! But it is clearly not specified regarding the wear of a women's under garments. So she was clearly out of here lane, addressing the issue, given the facts that you presented. Today's military have bigger situation to address.

I remember when my ship got to go to England for a port visit, when I got to the quarter deck I was told that I needed to change my shirt, it was a blue polo with white and orange stripes. After looking puzzled for a second the officer of the deck told me that my shirt was the color of the rival soccer team in town and I would likely get jumped or attack because of it, so obviously I changed my shirt.
So to answer you question it does depend on the context. If your state side and not doing any command related functions and are going home for the day you do you. If your in a foreign country remember that you appearance and actions affect how the world looks and treats the US. If you’re on base doing on base things you should conduct yourself accordingly you never know who is watching and how your appearance or behavior could effect your career.

Depends if it is blatant or not, hard to define, but you know it when you see it. Short answer, yes.

the question i have were they on base or off base?

Act like a lady in pubic and wear one and you will get more respect. I worked ina classroom at school after getting out of the Navy. A woman came to the High school to subsitute for a day. Everyone gossip behind her back about she did not have a bra on, but no one including me said a word to her about why the school and some employes thought she should not return.You really had to state to tell. I did not notice as you said, she was not wearing any see through clothing. Dress like a lady in pubic.

You have the right to question an order if it endangers life or property.

Yes. You can raise the issue with the Platoon Sergeant if you do it the right way -- I'm not arguing but I'm trying to understand so that if I'm in leadership position, I can make the right decision.

This is not a question here for RP. You would be better off asking within chain of command. In military a professional look is required off duty...(consult your regs)

My take on this is First Female PSG telling a Male Team leader to talk to a Female SM about wearing a bra. Gives a whole new name to delegation. 1st. This would be a good time for a written counseling protecting all involved, The thing that hits me the hardest Is "PSG I didn't Notice so maybe it would be better for you to approach said SM you have the details and answers she is going to want to ask about."
I did not see the violation if that is what it is by local policy, IMHO the PSG seen said violation she should have done an on the spot correction, Not put me in a position where I have to decide, or guess, or take sides, or take the flack for something I was never aware of. Which to me means I don't have a dog in the fight. In turn if I see said violation I would find another female SM and ask Her to set in on a Counseling just to so there is no misunderstanding. AR 670-1 does cover that with a blanket statement which Is "will present a professional appearance or Image", which can be interpreted as yes you need to wear said underwear.
In addition some common sense should apply. If you are wearing revealing clothing along with no Bra that's one thing but just because no one can see weather you are or not is not a good call in my opinion. "lets just say would not put myself in a position that I had to actually say I was looking that hard."

In closing I really don't think Rally Point is a good place to ask questions about this kind of thing your going to get more opinion than any thing. What I would say here is far different than what I would say in the civilian world. JMTC

I agree with SFC Johnston. I was out of the service for 11 years and learned what hurt my first time in service was not picking my battles, and not reading regulations...just believed what I was told. Take the high road and keep it at the lowest level possible. The down side is, 1SG's and CSM's are not required to have an open door policy, yet they are very good at seeing Soldiers when the Soldier needs it. Simply put, if you have to go to your 1SG or up to your CSM if that doesn't work. The last thing I can tell you, don't be afraid, all our supervisors still put their pants on the same as we do, one leg at a time....move forward and make sure this is a fight you want to have.