Why are stepchildren treated like 'whipping boys' ?

Why are stepchildren treated like whipping boys ?

A whipping boy is a boy who is blamed and punished for the failings of others.

I got beat up by my stepmom for every mistake of my stepsisters. They broke the window, I got beat up. They spilled coffee on the couch, I got beat up. They got caught smoking, I got beat up etc.
Even when they didn't do their homework, I got beat up again because my mom thought I was keeping them busy,
I took all these beatings in front of my snickering sisters.

Why are stepchildren treated like 'whipping boys' ?
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