Talk with the school counselor or with your pediatrician. Either of them can refer you to a therapist in your community who works with families. Even though your other daughter is not exhibiting the aggressive behaviors you describe, she is being affected by them and would benefit from family counseling.
It sounds as if teasing is a problem at your daughter's school. The counselor may be able to provide classroom lessons on reducing such mean behaviors and on how children can appropriately handle teasing when they are the targets.