Maybe you need to clarify.
If it's a situation where you are a named guests on the reservation AND everyone in the party (including your brother, his family and other named guests on the reservation) ALL want the
DDP it can be added, and you will pay for all guests named on the reservation. Sure, maybe they can help at the front desk letting you pay for part of the plan while other pay their share.......but the DDP will have to be added to the reservation for everyone in the party before check in. The DDP will not allow for you to be on the plan while your brother isn't. It's all or nothing for the party.