The fastest way to convert an older sibling into an enthusiastic teacher of Baby Sign Language is to have them teach the sign for brother or sister. Older sisters really want the baby's first sign to be sister. Once they have taught the baby that first sign, they will enthusiastically move onto the next sign.
We use sister when introducing our babies to the concept of family relations. If your baby has a sister, consider replacing the generic image on the flash card with a photo of her sister. When a baby has several sisters, we like to sign sister plus their first initial to mark the difference between sisters.