the second person
plural, "you guys", "you all", etc.
"Yins" is the second person plural, mutated by local usage from (probably Scottish) immigrant dialect. It's a compression of "ye ones" or "you ones," much like the American "y'all" comes from "you all". The possessive is "yinses" -- as in "Are dese Stillers tickets mine, or are dey yinses?"
I've heard "yins" used by my Western Pennsylvanian relatives, and every time I visit Pittsburgh or any cities in its vicinity--occasionally it surfaces as far away as Cleveland, where the locals include many Pittsburgh transplants. "Yins" and "yins guys" have a certain local pride attached to them--they mean the same thing to Pittsburghers that "y'all" means to someone from the South, and that "youse" and "youse guys" mean to someone from New Jersey; far from being a mark of shameful or "wrong" English, it's a badge of local pride.
I've heard--but not verified--that "yins" is also used in the U.P. of Michigan and other states primarily settled by Germans or Scots. I've even heard the word "yunser" or "yinser" used to mean "a person who speaks in the Pittsburgh dialect." Likewise, I've seen it spelled (most commonly) "yuns" or (rarely) "yens".