Is it permissible to give from Zakat money to the orphaned fiancée of one's brother to help her prepare for marriage, even though her paternal uncles are also assisting her with that?

Zakat is only to be given to those who are eligible to receive it. If the daughter is poor or needy and is not financially supported by any of her relatives, and she marries with a dowry, then it is permissible to give her zakat.