Son:  Papa, i like that girl and i want to marry her

Dad:  My son, don’t marry that girl, ‘coz she’s your sister and your mother doesn’t know 

The next day..

Son:  Mama, i want to marry that girl but father said, i should not because she’s my sister and you don’t know 

My son, marry that girl because your papa is not your father and he dosesn’t know