What should a brother do if his sisters do not adhere to all the conditions of the legitimate hijab and want to go out, and there is no one to take them except him, and they insist on going out even on their own, and they are not under his guardianship?

If sisters wear tight trousers without an outer covering over them, then the brother must explain to them the impermissibility of that and forbid them from it, and prevent them from going out if he can do so without causing harm. He should not go out with them while they are dressed in such clothes, because it involves cooperation in sin, unless there is an urgent need to go out with them, such as protecting them. In that case, he goes out to prevent harm.