As for informing them whether the money you are giving is zakat or something else, that depends on where the greater benefit lies. If it is better not to tell them, then do not tell them; and if it is better to tell them, then do so. What truly matters is whether they are eligible or ineligible. If they are not eligible, it is impermissible for you to give them anything; if they are eligible, then giving to them is prescribed. Whether you disclose or conceal that it is zakat should be guided by whichever course brings about the most benefit.

Disbursing zakat to brothers and sisters
Question: Is it permissible to give zakat to my brothers and sisters when they are minors, their father has died, and they have no income with which to live?
Answer: The maintenance of poor brothers and sisters is obligatory upon their wealthy relative. Since, as you have said, you are well-off while your siblings are poor and their father left them nothing, their upkeep is your duty. Therefore, it is not permissible for you to give them zakat, because what is required of you is greater than that—namely, to provide for them completely.

Disbursing zakat to a brother, grandfather, and grandmother
Question: I have a full brother who is poor, owns nothing, and is ill, and I am the one who supports and cares for him. May I give him my zakat? May I use zakat funds for his medical treatment? May I give zakat to my paternal or maternal grandfather or grandmother, given that I am the one who supports them?
Answer: This inquiry concerns his poor brother, and his impoverished grandfather and grandmother,