Is it considered an immoral act or one that could lead to vice for a brother to ask his sister to bring him his underwear while he is bathing, or for her to ask him for the same?

One should avoid everything that incites desires, especially in this era where provocations to lust have multiplied, and keep away from anything that gives Satan a path to a person. Souls are weak, and Satan flows through the son of Adam like blood.

Therefore, the door to Satan must be closed by preparing clothes before entering the bathroom to wash. The sister should do the same, or ask her mother or sister to give her clothes to avoid potential pitfalls.

The Prophet, peace and blessings be upon him, commanded to stay away from places of temptation, as exposure to temptation may lead to falling into undesirable consequences. Bathing naked may lead to inciting temptation and igniting desire, which could lead to falling into adultery, one of the greatest major sins, and it is even more severe with close relatives.