I love "yin"! I have a few friends from western PA and love when they're around so I can use "yinz" freely. Neither of them really use it in day-to-day speech, it's more of a shibboleth of sorts (or whatever the term would be for "look at me, I'm from group X"). Whatever, it's a fun word.

Yinz is literally the only evidence available that that part of PA isn't actually West Virginia.

I hadn't heard that one. My ex-father-in-law (from an Appalachian upbringing) was big on "you 'uns". "Yinz" reminds me of that.