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

DA PAM 670-1 goes more in depth of how we are supposed to be presentable both on and off duty. With that being said, I would definitely ask for all the information from your PSG and also look up the policy letters established by your command team.

Depends where stationed overseas. If in Europe, cool. If in UAE, Id be a little more discreet to show respect to the locals.

Interesting question. First and foremost, an active duty military personnel are always on duty, even when on your own time.

Since you were called on it, someone may have said something about how you dressed, which came across as unprofessional attire.

Nevertheless, there is a dress code for which you should have learned that in boot camp...

Yes. People think only the Regs matter. That is incorrect. Not every lawful order is covered in the regs and this is a lawful order

Get the DA pam 670-1 (Wear and Appearance of the army uniform). Be professional and courteous towards your chain of command. I ran into this while at Advance NCO Academy about wearing my beret in garrison. My company 1SG at ANCOC gave me the best advice after I got chewed out by the Academy CSM. Get the Pam and approach your team leader with it and show him/her the specific part of the Pam and if the Pam says so then hopefully your team leader will support your and then go to the PSG with Team Leader, etc. YOU will have to be strong and determined. DON''T give up. You go through your Chain of Command.
NOTE: I ended up going outside the chain of command with a couple of phone calls and TRADOC ended up calling the Advanced NCO Academy and told them in no uncertain terms that in garrison soldiers are allowed to wear their normal headgear in garrison, i.e. Recruiters wore their Smokey the Bear hats, Abn Soldiers wore maroon berets, SF wore Green. When we went to the field you wore the required head gear. TRADOC ended up threatening that the TRADOC Commander would visit the Academy and speak with CSM. The CSM then told us we were allowed to wear our headgear.
So if you're right you will have to be tough and stand your ground. Hopefully it won't go that far.
I can see your PSG just looking out for your welfare and trying to establish what she thinks is the best standards for another female.

You did not reveal where you’re currently stationed OCONUS. The local customs & mores have a great deal to do w/how you present yourself in public is considered acceptable, & whether or not your public appearance may present a problem for your command, BUT ALSO, whether or not it presents a danger to you as a female soldier. I also have to ask, have you sat down w/your platoon Sgt in an informal & private setting & asked her to please clarify what the perceived problem is? An adult conversation, one to one, will give you information, & hopefully, as well, help your platoon Sgt see you as a mature young woman. If it turns out that a bra is the answer for all the problems in your current part of the world, say “eff it” to yourself, put on the sports bra, & get on w/life.

Your Sargent is approaching dangerous sexual harassment ground. Accept his order but report his action when safe to

Well first of all your team leader is weak on how this was approached. If your team leader has a disagreement with the PSG the two should have had that conversation. Terrible leadership to say the PSG wants you to do this instead of just stating it for themselves. Basically team leader is saying I dont agree with the PSG but She wants me to tell you to wear a bra. Why have a team leader? With that said many regulations are written in a way that gives the reader and enforcer there own interpretation. Its not always black and white. I agree with 1SG Johnston choose your battles, is this one worth the headache, only you can decide. Me, I choose mine carefully.

From my memory on Military rules and regs, it would be an appropriate instruction, even off-duty, if the civilian attire is being worn on the confines of a military installation.

On the other hand, if the civilian attire is being worn outside the confines of a military installation (in the local Walmart, a local movie theater, etc.) then I would agree that the instruction was inappropriate.

If you are on post yes. Just like males can’t wear wife beaters