They don't always, but a good many of them normally do so because of how society is taught to view females, and females in their own family.
Some do so out of pure concern in their sister taking on much more than she can handle in any case, so rather than expecting her to be able to handle anything coming her way and being disappointed, if she messes up or has a hard time, underestimating softens any blow because they feel it was expected, and if she succeeds, then it's a pleasant surprise.
However, some do so out of spite in believing females have a certain place they should stay in, and can't be expected to move above that place in any case at all. Females in their family just seem the easiest to control, since strangers feel no obligation or connection to them, and a sister may feel oddly guilty for accomplishing what her brother(s) didn't want her to.
Zayn has three sisters and no brothers Liam has three older sisters and no brothers Louis has four sisters and no brothers Harry has one sister and no brothers Niall has no sisters and one brother
Maybe they are bored of the same sex and do things different
No he always wanted to be the only child.
He had 11 brothers and sisters.
Sometimes a brother does understand the feelings of his sister. But not always.
Mabye cuz they want to feel safe with a boy
There are 4 brothers and 3 sister's all up. Assume your one of the brothers - hence you have 3 brothers and 3 sisters. However any one of your sisters, would have 4 brothers and 2 sisters.
no she has no brothers or sisters.
yes, 7 brothers and sisters
he had 2 brothers and 3 sisters
She had 12 brothers and sisters
Athena had no brothers or sisters.