@Seraphinaf5 12 would be the answer but could be wrong if the sons are from diferent mothers & the mother got maried some where else and got the Dota from there, the sister of the son is not a child of the man in question.
@kirya_ug B4 u find out why parents take kids 2 single schools, first find out why they start/operate single schools.
Finish section A b4 you go to section B.