gaw: I am not sure what you mean by “prove it”. If you are talking about allowing a woman to nurse a child in a non-tznius manner here is a quote from the “tshuvah” of Rabbi Katz of YCT:
??? ?????? ????? ????? ???? ???? ????? ?????? ?????? .
??? ?? ?? ???? ???? ???? ????? ????? ???? ???? ?????, ??? ??? ???? ????? ????? ???? ????? ???? ?????, ???? ???? ??????? ??? ?? ??? ????.
So we see that he is is stating that a woman can have herself exposed for a number of minutes since to herself it is not an “ervah”.
So yes, IMO that is the definition of “in a non-tznius manner”.