The term "child" shall include a posthumous child, a child legally adopted prior to the injury of the employee and a stepchild or acknowledged illegitimate child dependent upon the deceased, but does not include married children unless wholly dependent upon him. "Grandchild" means a child of a child. "Brother" and "sister" include stepbrothers and stepsisters, half-brothers and half-sisters and brothers and sisters by adoption, but do not include married brothers nor married sisters unless wholly dependent upon the employee. "Child," "grandchild," "brother" and "sister" include only persons under eighteen years of age or wholly dependent upon the employee.