The butler who ran out was the one who hired the girl. So there was suspicion he was involved.
Then inside the MC's room they found a letter hidden under her bed, FROM THE BUTLER.
The short version? Whoever told the girl her brother was sick was lying, the brother is healthy.
He also know there's a good chance the kind of comments like so many we see here would come up and if her family, being commoners that they are, get punished for it, whenever the MC return and find out she would be very sad.
So in order to prevent that, he is going to do what he can do, and end with apology for doing things on his own.
The brother and dad also decide to do the same, to make sure that whenever the MC return, she won't be greeted with more tragic news.