What is the ruling on giving a small monthly sum to one's sister as assistance, when she is of moderate means?

If the money is not from zakat (definition), then providing assistance to one's sister is permissible and desirable, as it falls under the category of maintaining family ties (silat al-rahim) and gift-giving, which fosters affection, in accordance with the Prophet’s (peace be upon him) saying: "Give gifts, and you will love each other." However, if the assistance is from zakat funds, then it is a condition that the sister must be among those eligible to receive zakat, such as the poor and the needy.