Sheikh `Abd Allah Nâsir al-Sulamî, professor at the Higher Juridical Institute
If someone wants to pay his Zakâh to one of his relatives who is needy then it is lawful, as long as he is not already responsible according to Islamic Law to take care of that person. A person may therefore pay his Zakâh to his sister or brother.
The close relations who are not eligible for Zakâh are the parents, grandparents, children, and grandchildren, because we are obligated to spend on those people in any case. The sister is different, as the Malikî school of thought states that the man is not obligated to spend on his brother or sister.
And Allah knows best.
Source: Islam Today