B. Any person who performs or causes to be performed cunnilingus, fellatio, anilingus, or anal intercourse upon or by his daughter or granddaughter, son or grandson, brother or sister, or father or mother is guilty of a Class 5 felony. However, if a parent or grandparent commits any such act with his child or grandchild and such child or grandchild is at least 13 but less than 18 years of age at the time of the offense, such parent or grandparent is guilty of a Class 3 felony.