Is it permissible to give zakat to a married sister who has children, whose husband is retired and has a low income?

If your sister's husband is unable to provide for her and their children, then she has more right to your Zakat than others. By doing so, you will have two rewards: the reward for Zakat and the reward for upholding kinship ties, based on the Prophet's (peace and blessings be upon him) saying: "Charity given to the poor is a single charity, but charity given to a relative is two: charity and upholding kinship ties."