An employee's spouse; children who are unmarried and under age 21 years or who, regardless of age, are physically or mentally incapable of self-support; dependent parents, including step and legally adoptive parents of the employee's spouse; and dependent brothers and sisters, including step and legally adoptive brothers and sisters of the employee's spouse who are unmarried and under 21 years of age or who, regardless of age, are physically or mentally incapable of self-support.