I say, and with God's success: The private parts of a woman in front of her non-marriageable relatives are between the navel and the knee, including the abdomen and back. The chest, meaning the breasts, is not considered private in front of non-marriageable relatives unless it causes temptation. If it does cause temptation, it is not permissible. This is because the breast is a part used for nursing infants, and sometimes a woman needs to nurse in the presence of her children, siblings, and father, so it is allowed. Typically, there should be no exposure without reason, and God knows best.