What topolects distingish between 娘 and 孃? Why?
孃 usually is used by people who live in southwest China, to refer women who are in the middle age. People in southwest China use it to call their older female relatives, or female strangers who seemed like in the middle age.

娘 means mom, but it usually used by people who live in North China. In South China, people call their mom as 妈妈

I'm a Chinese, and these are my experiences in using Chinese. Hope it helps.