What is the ruling on a brother helping his non-hijabi sister bring her immodest clothes from the outdoor clothesline, or bringing all clothes, including the immodest ones, into the house at their mother's request?

It is permissible for a brother to bring clothes for his sister, whether she requested them or he did so on his own initiative, if she will wear them at home or go out in them. However, if he knows that she requested them to go out in them, and they are not religiously permissible, then he is not allowed to assist her in that, because it is aiding in disobedience. This is based on the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." He should also teach his family and guide them to goodness, enjoin them to what is right, and forbid them from what is wrong, so that he may attain a great reward.