Question 236815
The answer is 6 children.
The first 4 are given by the question.  
Now, if you're a boy and add yourself to the family, you'd have 2 brothers and 2 sisters.  The same thing would apply to other two brothers, right?
However, the girls could only count 3 brothers and one sister.  So, in order to satisfy the problem, you also need to add a girl to the family, so each of them can say they two sisters, in addition to their brothers.