Is a person sinful if he does not fulfill his brother's oath upon him, and is it obligatory to inform him if he breaks his oath, especially if he is the type who would not fulfill his oath if he were to break it?

It is recommended to fulfill a brother's oath, unless there is a benefit in not fulfilling it, as the Prophet, peace and blessings be upon him, said: "The Messenger of Allah, peace and blessings be upon him, commanded us with seven things," and among them he mentioned, "...and fulfilling the oath of one who swears."

There is no sin upon you if you cause him to break his oath, unless there is a sin or corruption in doing so.

If you decide to cause him to break his oath and you fear his reaction and punishment, you may follow the opinion of those who permit atonement on behalf of another without their knowledge. In that case, you can perform the expiation on his behalf without informing him.