All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Your brother has to give up the idea of marrying this girl because of her bad reputation. He is required to be obedient and dutiful to his father. He should look for a religious and virtuous girl with a good reputation to marry. You are all required to offer him advice in this respect. However, if he declines to accept your advice, your father, if he thinks that it is beneficial, is entitled to desert him and order you to do the same, for he may be deterred and give that girl up. You should obey your father in this, if you believe that this is likely to stop your brother from doing what he wants. This should be done if the girl indeed has a bad reputation, not just rumors. In such a case, desertion is not considered a form of severing kinship ties.
Allaah Knows best.