Why families assign the scapegoat role, and why it usually has more to do with what you noticed than what you did wrong.
Once a woman figures out she was the family scapegoat, the next question she asks is why it was her and not her brother or sister. A child who gets blamed while a sibling gets patience comes up with the only explanation available to her, something about her was wrong from the start, and a lot of grown women still believe it. If she'd been easier or less trouble, she tells herself, her parents would have treated her the way they treated her sister.
Parents pick a scapegoat for reasons having to do with the parents. A father who can't stand feeling wrong or a mother who's miserable in her marriage needs someone to point at. A child is the easiest person in the family to point at, because she can't leave and can't argue her way out. Two parents who fight constantly can agree on one subject, their daughter is the problem, and for as long as they're agreeing about her they aren't fighting about money. A father's gambling or a mother's affair gets explained away by pointing at the child who acts out, and nobody asks what she's reacting to.
The Self-Fulfilling Prophecy
It starts with a parent watching one child more closely than the others, expecting to find something wrong with her, and a child watched like that gives the parent plenty to find. A toddler who cries at bedtime or a nine-year-old who sulks about a canceled birthday party is behaving like any child her age. A sibling who does it gets a hug or gets ignored. In the targeted child it gets read as proof she's selfish or difficult. The parent starts treating her as though it's true, criticizing her, trying to control her, then going cold on her for days. She gets angry and defiant under that much scrutiny, anyone would. Her anger becomes the evidence her parents and siblings point to for the next thirty years.
Her brother grew up with parents who weren't watching him for flaws, so when he says their childhood was fine, he's describing the childhood he had. His memory and hers can both be accurate, because their mother was patient with him and searched her room every week.
Timing and Inheritance
Some of the reasons a parent lands on a particular daughter come down to timing. She was born the year her father was laid off, or she came second, after an older sister had already been made the good one. She might look like the ex-husband her mother still hates, or have the temper of the grandmother her father hasn't spoken to since 1994. A daughter who won't wear dresses to church, or who has a learning disability and needs more help with homework than her parents want to give, gets picked for reasons a parent wouldn't admit to anyone.
The Crime of Noticing
The other common reason is the child noticed things and said so. She was the ten-year-old who asked why the rent money kept disappearing, or the teenager who didn't laugh when her mother made a joke about her aunt's weight at Easter.
A family depending on nobody mentioning the father's drinking has no use for a child who mentions it.
The fastest way to make sure nobody believes her later is to establish early that she exaggerates and makes things up. By the time she's old enough to tell a teacher or an aunt what goes on at home, the aunt has heard for years she's dramatic.
The Projection Test
A lot of the time the accusation describes the parent, and the mother who calls her daughter selfish is the one who spent the grocery money on herself. Parents who shut down their own feelings decades ago get irritated by a child who gets excited about things and says what she thinks. Plenty of scapegoats were punished for their honesty and for how much they felt, and their parents called both of those selfishness.
If you want to test this on your own family, write down the word your parent used for you most, selfish or ungrateful, and look at whether the word fits the parent better than it fit you. Then write down what you noticed as a child that the rest of the family acted like they didn't see, the drinking or where the money went, because there's a decent chance the noticing is why you were picked.
Further Reading
If you are trying to understand the mechanics of the family scapegoat role and how family systems use one person to avoid looking at their own dysfunction, exploring the entire family structure can provide clarity.
Explore Family, Interrupted