-Removed-it's not child benefit that's capped at two children. It's the Universal Credit child element. UC is means tested, and parents on it are economically encouraged to not have more that 2 children. If only all parents could predict the future, they're all being encouraged to know, should something happen (like their husband leaves, for example) that, if they one day need income based benefit, they'd be wise to only have two children.
Say your brother's had four children with his wife. All great, both working. Then he gets ill and can't work, that's OK, he's paid NI and he gets New Style ESA and she keeps working. He's looking after the kids while she's working. But he starts needing care. So she gives up work and looks after him. They claim benefits, carers allowance for her. It's not enough, they've got kids, so they claim Universal Credit too.
But, oh dear, they've got 4 kids, not 2. Suddenly their kids are growing up in poverty. Not their fault. Not your brother's fault. Not his wife's. Why can't we expect the benefit system pick up the cost of raising these children?