Answer: 3 sons and 4 daughters.
Easy explanation:
Think about what each child sees.
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If there are 3 sons, then each boy sees 2 brothers. The riddle says each brother has twice as many sisters as brothers, so he should see 4 sisters. That means there must be 4 daughters.
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Check the daughter rule: each girl would then see 3 sisters and 3 brothers — the same number, which matches the riddle.
So the family has 3 sons and 4 daughters.
