Is it permissible for a young man and a young woman to communicate when they are joined by forbidden fosterage (rada'a muharrama), knowing that the girl's mother forbids this communication, and is it permissible for the girl to deny this communication or swear that it has not occurred when questioned by her mother?

It is not permissible for a girl to disobey her mother regarding what she has forbidden her from, especially if there is suspicion or temptation in the communication. Connecting with a foster sister is desirable, not obligatory. It is appropriate to refrain from communicating with the girl due to her mother's prohibition and any potential harm that might result from the communication. If communication does occur, the mother should not be informed. If she asks, one may use equivocation without outright lying.