While in Create-A-Sim at the start of the game, click the bottom at the bottom to add a second Sim. When you're done and exit Create-A-Sim, a family portrait will appear where you give the family their last name. Click on one of the Sims and drag the icon over to the other Sim. You will then be asked to choose the relationship. Select Siblings and they will be brothers.
This does not need a mod, so I'm moving this thread to the Miscellaneous Help board.