What is the best way to deter girls who browse inappropriate websites and forums, and is tracking their devices and informing their mother about it considered a sin?

It is not permissible for a Muslim to spy on his brother or to seek out his faults and slips. Whoever does so will be exposed by Allah, for the Messenger of Allah, peace and blessings be upon him, forbade that, and we have been forbidden from suspicion and spying. Moreover, a Muslim's faults should be concealed.

However, if strong evidence exists that gives rise to suspicion, there is no harm in investigating with the intention of offering advice and saving the Muslim. This is not considered the forbidden form of spying, but rather a verification for the purpose of offering advice. Whoever becomes aware of such a matter should deal with it with wisdom and good admonition, and it is permissible to inform someone more capable of offering advice, such as the parents.