Is it permissible to give charity to an older brother who is unemployed, without his knowledge, in order to save it for him and prevent him from spending it indiscriminately?

It is permissible, rather, it is recommended for a brother to give charity to his brother if the latter is deserving, for he is more entitled to his charity than others. This is because charity given to a relative is both charity and an act of maintaining kinship ties. If one sees a benefit in delaying the payment of voluntary charity until a specific time, there is no harm in that. As for obligatory Zakat (definition) on wealth, it is not permissible to delay its disbursement after its due time, except for a valid excuse.