The character used here to represent ‘niang’ is 娘 but that obviously can't be the etymologically correct character for the above meanings (i.e.: what, how, etc.).
- Etymologically Correct Character For The Sichuanese ‘niang’ Meaning "What"?
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There is a better link to the article that is quoted above in this post on Sina Blog, which will let you search the whole article in your browser.
The article mentions the use of 合音字 in Qionglai and related Sichuan dialects, i.e. one character writing two syllables. Examples the author gives include “不晓” 写作 “表”，and “那样” 写作 “浪”. Possibly 娘 is simply a 合音字 for the interrogative form 哪样.
Here's a partially supported theory. I hope someone comes up with better evidence:
什么into 成都话 is
啥(子) = sa˩˧ tsɿ˥˧.
娘in question is actually
哪in a certain context. For evidence, I give the following example.
哪个样(子) = naŋ˥˧ ko˩˧ iaŋ˩˧ tsɿ˥˧.
哪is only in some circumstances pronounced
My source is a "freely" available copy of 汉语方言词汇. Note that it's not searchable and is therefore somewhat tedious to use. I don't speak any variety of 四川话 and I have not found corroborating resources.