Is it permissible for a woman to wash and iron her husband's brother's undergarments and arrange them in his room in his absence, knowing that she helps her mother-in-law with household chores?

It is permissible for a woman to wash her brother-in-law's clothes and arrange them in his room in his absence, with her husband's permission, as long as there is no trace of a wet dream on the clothes; in which case, the brother-in-law should remove it out of modesty. This is supported by the hadith narrated by Aisha (may Allah be pleased with her) concerning modesty regarding the trace of a wet dream.

Regarding a wife serving her husband's family, Ibn Uthaymeen stated that she is obliged to serve her husband in a reasonable manner, such as washing clothes, cooking, and sweeping the house. As for serving the husband's parents, she may be obligated if she knew of their presence and their need for service at the time of marriage and did not stipulate otherwise. The matter is less strict concerning the husband's grandfather, unless they live in the same house with them.