The rules of Walt Disney World concerning the dining plan is that you may not use your dining credits to purchase meals for anyone who is not registered on your reservation.
If you have, for example, two adults and two children on your reservation (this will be listed on your key card), you will be able to use the dining plan to purchase a maximum of two adult and two child meals in one transaction. You will not be able to buy a third adult meal for your brother and use your dining credits to pay for it.