Will the questioner be complicit in sin if she accompanies the girl with whom her brother is dating, with the knowledge and consent of the girl's family, or is it better not to communicate with her until after the official engagement?

There is no harm in speaking to a girl and befriending her, and it is not considered participation in sin. However, it is obligatory to advise her and the brother to adhere to the limits of the Shari'ah in the relationship. The suitor is a stranger to his fiancée until the marriage contract is concluded. Thus, it is not permissible for him to be in seclusion with her, touch her, or engage extensively in conversation with her without need.