In MSM 娘 and 孃 are usually considered one and the same, where 孃 is Traditional and 娘 is simplified. These two characters can practically be used interchangeably. In Southwestern Mandarin, however, a split has been maintained.
It should be a little something like this:
娘 = 阳平 = mom
嬢 = 阴平 = woman
I'm not sure if this a special feature of SWM or a remnant of something else. Do other topolects do this? Is there a reason for this split?