What is the ruling on a sister refusing to perform some household chores, such as laundry, cleaning, and preparing food for her brother, claiming that she is not obliged to do so, and that what she sometimes does is sufficient?

A sister's kindness to her brother, such as cooking his food, washing his clothes, and so on, is an act for which she will be rewarded. It is considered a praiseworthy act of upholding kinship ties. She should not disdain this reward, especially if it does not harm her. However, it is not obligatory for her, and she does not sin by not doing it.