They consist of Dudley, Vernon, and Petunia. They also have another child, whom isn't theirs, named Harry. But, see that broom? Where does it come from? A servant, no. But close enough to one.
Petunia's sister's adoptive child. If you were wondering, yes, she is abused. VERY much, too. "Then why didn't they shave her head?" That's a wig. They couldn't bare the sight of her without one, but she likes hair, and they like her miserable.
But not in a good way. She's forced too. Or else, physical punishment. But, she gets that anyways.