Is it permissible to give Zakat al-Mal (obligatory charity on wealth) to help brothers and sisters with marriage expenses and costs, as well as to meet non-essential family demands like home renovation and purchasing luxury items?

There is no objection to giving zakat (definition) to siblings who cannot afford the costs of marriage if they are poor. Zakat given to a needy relative is both charity (sadaqah (definition)) and a bond of kinship (silah). As for assisting family members with zakat for non-essential needs, it is not permissible if their maintenance is obligatory upon her (the zakat giver), because in doing so, she would be saving her own money. However, if their maintenance is not obligatory upon her and they are in need, then it is permissible to give them zakat to the extent of their sufficiency for food, clothing, housing, and all other necessities, in a manner befitting their situation without extravagance or stinginess. The desire for outward display and luxury is not a necessity. A poor person eligible for zakat is one who cannot find enough to suffice himself and his family for a year.