Can I give Zakah funds to my sister who is not married?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Giving Zakah to brothers and sisters is allowed. It is even preferable provided they are poor and their expenditure is not compulsory on the person.  The Prophet (Sallallahu Alaihi wa Sallam) said: “Sadaqah (charity) given to a poor man is just Sadaqah, but when it is given to a poor relative it has double reward; reward of Sadaqah and reward of kinship.” [Ahmad, al-Tirmizi and he graded it as Hasan]

If you are obliged to spend on them then you are not allowed to give them from your Zakah since spending on them is compulsory.

If your sister is poor and does not have a father or son to spend on her then you are obliged to spend on her.

Allah knows best.