Many children have female siblings, sometimes older and sometimes younger. Teach your baby the sign for the word sister in baby sign language with the following description and video.
Hold one hand in the shape of the letter L in front of the chest with the thumb pointing up. With the other hand, begin with the sign for girl. Form the hand into a fist with the thumb sticking up. Rub the thumb on the face from the cheek to the chin. Then form the hand into a signed letter L. Tap the second L on top of the first L.
Beyer, Monica. 2006. Baby talk: A guide to using basic sign language to communicate with your baby. New York: Jeremy P. Tarcher/Penguin.