The scapegoat is the child a family agrees is the problem, and once that's settled, most things that go wrong get attached to her. It's rarely stated outright. It shows up in who gets called when there's a fight, who's brought up as an example of what not to do, and whose version of events nobody checks.
Which child gets it has less to do with behavior than people assume. It often goes to the one who said something out loud, or who was harder to manage as a toddler, or who looks like a relative nobody liked. Once it's assigned, it holds, and it holds into adulthood even when everyone involved is over forty and the original incident was in 1998.
The posts here cover how the role gets assigned, why siblings go along with it, what happens when you stop arguing your case, and what it's like at holidays when everyone still knows their part.
There's a call some people make whose only point is to make sure you know that they know. No help attached, no comfort, just the letting-you-know. On the relative who watches everything you do and supports none of it, and what to do once you see what the watching was for.