Is it permissible to give Zakat on wealth to relatives, and is Zakat al-Fitr considered invalid if the receipt from the charity organization does not state that it is Zakat al-Fitr?

It is permissible to give zakat (definition) to relatives whom one is not obligated to support, such as brothers and sisters. In fact, this is more meritorious as it combines charity with upholding kinship ties. As for Zakat al-Fitr, it is sufficient to authorize a trustworthy organization to disburse it, and it is not a condition to name it "Zakat al-Fitr" on the receipt. It is preferable for the payer of zakat to distribute it themselves if possible, otherwise, they may give it to trustworthy organizations.