Why brothers always underestimate their sisters?

Answer 1

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.