How can I help this family now that I have come to see her as a sister?

Marriage is subject to the five legal rulings. If the marriage is with someone for whom it is legislated, then obedience to the father in it is obligatory, because showing piety to him is not complete except by obeying him in what he commands, provided it is within one's ability, does not contradict the Shari'ah, and does not cause harm. However, if he commands marriage to a specific woman whom one does not desire, then obedience is not obligatory. The "good" (ma'ruf) in which obedience to parents is obligatory is everything permitted by the Shari'ah, as long as it does not involve harm. If it is forbidden, then obedience is not obligatory, because "Obedience is only in what is good." It is obligatory to stay away from a girl with whom one has a friendship, but it is permissible for him to help her do good deeds within the limits of the Shari'ah.