Is the one who remains silent about her brother's relationship with a girl she dislikes, as sinful as they are?

Your brother's relationship with the girl is forbidden, and they must repent to Allah. You must advise them and forbid them from this evil. It is not permissible for you to assist them in this forbidden relationship. You must repent for what has emanated from you, and promptly denounce this evil and change it as much as possible. Do not concern yourself with what may result from this, such as your brother's displeasure or ill-treatment, for pleasing Allah is more important than any worldly benefit. Remember Luqman's saying to his son: "And enjoin what is right and forbid what is wrong and be patient over what befalls you."