Is it permissible to give charity to unemployed sisters or those who are still studying, or to use it for household expenses to relieve the burden on the father, or must it be given to a needy person?

Your needy relatives are more deserving of voluntary charity from your salary, based on the Prophet's (peace be upon him) saying to Abu Talhah when he wanted to donate his garden: "I think you should give it to your closest relatives." And based on his (peace be upon him) saying: "Charity given to a poor person is one charity, but charity given to a relative is two: upholding kinship ties and charity." The most deserving among them is a needy father, because he provides for the household. If he is not needy, you may give it to your mother, your sisters who do not work, or others, and you will be rewarded in any case.