How can the eldest brother return the money he took from his siblings' inheritance ten years ago without their knowledge, and is it permissible for him to claim that the amount is from a benevolent donor to avoid their reaction?

He is not obligated to inform the heirs of the reason for the amount taken from their father's inheritance. It suffices to deliver it to them while informing them that it is their right. The intent is for them to know that it is their right, not how it was taken, especially if informing them would lead to estrangement or harm.