Is it permissible for one who trespassed against his two brothers’ property and returned it to them as a gift, without informing them, to suffice with that, or is it obligatory for him to inform them?

It is sufficient for the thief to return the stolen goods to their owners without informing them of the truth of the matter, if he fears scandal or harm. He can circumvent this by sending the money or its value with a message, without mentioning his name, indicating that this money is rightfully due to its owner from a person who has repented to Allah. He must seek Allah's forgiveness, repent to Him, regret his action, and resolve not to return to it.