Is it permissible for a girl to marry the younger brother, knowing that she had a relationship "as close friends" with his elder brother, who is unaware of their relationship?

There is no objection to a girl marrying her fiancé if she has repented from the forbidden relationship that led to adultery, and on condition that there is no fear of her resuming her relationship with his brother. If it is known that the relationship continues or there is a fear of its resumption, the marriage is prohibited as advice to the suitor that she is not suitable for him. However, if it is known that the relationship has ceased and there is no fear of its renewal, then there is no objection to the marriage.