To sign brother, make both hands into an 'L' shape with your thumbs and index fingers extended. Hold your non-dominant 'L' hand down by your chest. Take your dominant hand and starting at the forehead, forming the tip of a baseball hat, bring the hand down to rest on your non-dominant hand, transforming it into an 'L'.
When your baby is getting started, brother gets simplified a lot - often the 'L'-shaped hands will look more like fists, and your baby will often only do the motion with one hand.
Getting an older sibling involved in teaching your baby sign language is a great way to get both children engaged. Older brothers love to teach the sign for brother. Have them make the sign whenever they are around your baby. Siblings are often highly motivated to get the baby to sign their name first and will put in a lot of work with the baby to make it happen - so don't be surprised when the baby's first sign is brother or sister.
We use the sign for brother when we introduce our babies to family dynamics. Consider replacing the generic image on your flash card with an image of your baby and his or her brother.