There are two possible ways for a boy to call a girl a smelly sister. First of all, this title may be a kind of intimate title, expressing the love and concern of boys for girls. Even if the girl is a little headstrong or bad-tempered, the boy still wants to use this name to express his special concern for the girl. Secondly, if both boys and girls are in their late teens and early twenties, and boys are full of hormones, he may use this title to attract girls' attention. Boys think girls are cute in their hearts. Maybe girls have some funny characteristics. This kind of address can show boys' different feelings and attitudes towards girls.