Isn't the wife of the brother.

Like the wife of the son or the wife of the father.

The wife of the brother, the brother of her husband is not a mahram for her.

So she must cover herself from him just as she covers herself from a non-mahram man.

In fact, if it were said that she must cover herself from her husband's brother even more than from a man on the street, if that were said, it would have some basis.

Because if she uncovers her face in front of her husband's brother while he is with her in the house.

The fitna becomes greater, meaning when he enters the house and Satan has tempted him, he might try to commit a sin.

But in the street, if a man looks at a woman, he cannot do anything.

So the important thing is that the Messenger ﷺ pointed out this issue.

He said: Beware of entering upon women.

They said, O Messenger of Allah, what about the in-laws? He said: The in-law is death.

Meaning, be very cautious.

Do you see the answer now? So, the brother of a woman's husband is not a mahram for her.

The father of a woman's husband, is he a mahram or not? He is a mahram.

The son of a woman's husband.