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

Be modest .. respect others and definitely

It never ceases to amaze me; the never ending stream of people who enlist and still have no clue to what they have agreed.

If a male can go shirtless, there is no reason a female cannot go shirtless because of some archaic association of the female breast as a sexual object or indecent. So not wearing a bra shouldn't even be an issue. Ridiculous! The U.S. 10 Circuit Court of Appeals agrees. Time for the rest of the world to catch-up. #FreetheNipple

If a female soldier wants to show her tities more power to them tities

Have you ever heard any leader tell a male soldier to go put a pair if underwear on? I havent. Udually nothing but laughter when it was seen or heard the guy wasnt wearing any.
It's all relative!

This is a strange thing as when I served 1968-1969, most of the troops were male. The only females were nurses. Hey it is what it is. I really do not know what to say. I am totally out of my element here

You might just want to take her advice and wear a bra, she likely just trying to help. And say ‘thank you sergeant’.

What do the regs say? What are local cultural and religious mores like? Have you requested to discuss the matter at the next highest chain of command level? Is this fight worth fighting? Who was King Pyrrhus of Epirus?

Ok after reading the comments posted and your further clarification on the situation I gather the following: Plt sgt is female, discussed and instructed 3 Junior to her/senior to you team leaders in her decision in regard to the uniform regs pertaining to off duty attire by females and you question "Was this a lawful order?". Answer is yes, "Should she have had the male team leaders pass on her Plt sgt decision about the matter?". Answer again is "Yes". They need to understand her platoon policy. No mention is made of this being addressed due to the Plt Sgt getting the 'word from above' to do so, or if this was in response to Command Policy due to where you are stationed overseas or how long the Plt Sgt has been a member of that Command. Take the personal 'feelings and opinions" out of the situation and you might recognize the situation (bras worn in civ clothing in that area of the world) might be one she may have recently been told to 'resolve' at the lowest level of command (all of hers - which means she needed to discuss with and inform her team leaders)

The short answer is.....YES. Perhaps your breastworks need additional fortification.