For starters: Shu"A Orach Hayim 75:
"להזהר מגלוי שער וקול אשה בשעת ק"ש וכן שלא לקרותה כנגד הערוה. ובו ו סעיפים:
טפח מגולה באשה במקום שדרכה לכסותו אפי' היא אשתו אסו' לקרות ק"ש כנגדה:
הגה וי"א דוקא באשתו אבל באשה אחרת אפי' פחות מטפח הוה ערוה (הגהות מיימוני פרק ג') ויראה מדברי הרא"ש דטפח באשה ערוה אפילו לאשה אחרת רק שבעצמה יכולה לקרות אע"פ שהיא ערומה כדלעיל סי' ע"ד:
A handsbreadth that is uncovered on a woman in a place where it is her practice (lit. way) to cover it, even if it his wife, it is forbidden to read the Shema in front of it.
And some say that this is specifically with respect to his wife but with another woman even less than a handsbreadth is Erva (lit. nakedness). And it seems from the words of the Rosh that a handsbreadth on a woman is Erva even to another woman, except that by herself she is able to read [the Shema] even though she is naked like was said earlier in Siman 74.
Seemingly, the Mechaber does not set a fixed definition of the Ervah (for the purpose of Krishma), but relative to the currently accepted norms, so the chest in our culture would have the severity of the Makom Ervah.
However Rem"a adds that this is relative to men, but between women, only Makom Ervah is considered to be unacceptable for reading Krishmah.