Is a sister who commits adultery over the phone considered an adulteress, and what should be done about her after informing the mother without success?

What your sister is doing—talking with a foreign man about explicit topics over the phone and exchanging messages with him that are only appropriate between spouses—is forbidden (haram). This act is considered a form of the adultery of the ears and tongue. The Prophet, peace and blessings be upon him, warned against this, saying: "Indeed, Allah has decreed for the son of Adam his share of adultery, and he will inevitably attain it. The adultery of the eyes is looking, the adultery of the ears is listening, the adultery of the tongue is speaking, the adultery of the hand is seizing, and the adultery of the foot is stepping. The heart yearns and wishes, and the private parts affirm or deny that."