Is it permissible to give Zakat on money to the husband's sister to cover part of the cost of airline tickets for her and her son to visit her family in Brazil, especially since she has been unable to visit them due to financial hardship?

It is permissible to give Zakat (definition) to the husband's sister if she is poor or needy, has no provider, or if her provider does not provide her with sufficient maintenance, housing, and basic needs. It is also permissible to give her Zakat under the category of wayfarer (Ibn al-Sabil) if her basic needs are met, and all she lacks are the travel expenses to visit her family. This is because Ibn al-Sabil includes anyone who intends to undertake a permissible journey for a legitimate purpose.