Um, why did the guys dress up as girls in chapter 9?

To be in a disguise.

He refused to go, his sisters demanded he go, so the compromise was the disguise.

Oh.. So it being a masquerade wasn't enough of a disguise already

But wait, didn't his brother dress up as a girl too? What's his reason?

Do you think the brown-haired sister is a boy? He had an older brother (who became the duke and hates MC), a blond sister(who teaches at the academy), and a brown-haired sister (she's the knight).

Oh, right. I mistook her for a guy lmao. I thought she ripped off her wig too after she got into the carriage.