wives of brothers

妯 is composed of the radical of 女 (woman) and 由 (reason). 由 depicts oil droplets congealing into fat, with the original meaning being oil or fat, and by extension conveys the idea of harmony and smoothness; thus 妯 conveys the idea that just as brothers are like the limbs of one body, the wives of brothers must also relate to each other with harmony and smoothness for the family to be at peace, means the wives of brothers, or sisters-in-law.