So I had to look that up. I’m not from Pittburgh and am not familiar with the dialect.
This is what I found, which I am sure you are aware of, but it’s kind of a neat explanation:
The following therefore is the most likely path from you ones to yinz: you ones [yu w?nz] > you’uns [yu?nz] > youns [yunz] > yunz [y?nz] > yinz [y?nz]. Because there are still speakers who use each form, there is no stable second-person plural pronoun form in southwest or central Pennsylvania—which is why this pronoun is variably referred to or spelled as you’uns, yunz, yinz, yins or ynz.
Yinz is often coupled with the word guys in conversation (equivalent to you guys), as in, “Are yinz guys gonna watch the ball game?”