Is it permissible for a brother to impose the hijab on his two sisters who do not wear it, and to spy on their phones under the pretext of protection, especially when their parents are still alive? And is the brother sinful if they do not wear the hijab?

The father is primarily responsible for his daughters. He must educate them, discipline them, and prevent them from displaying their adornments. He should first try to change them with words, then with his hand if they do not desist, by preventing them from going out without a legal hijab (definition). As for the brother, he should object verbally and gently. He is permitted to shun them if that benefits them. He is not permitted to spy on them unless it is to prevent an evil when suspicion arises.