Answer:
From the reference frame of one of the sisters, there are n sisters and n brothers.
However, from the reference frame of a parent, we see the sister did not include herself in the counting.
This means that there are n boys and n+1 girls.
From the reference frame of one of the brothers, there are n-1 brothers (since he wouldn’t count himself), and n+1 sisters. Also, each brother sees k brothers and 2k sisters.
As a result, we can combine these deductions to create the equation 2(n-1) = n+1. Solving, we find n=3.
Final Answer: There are 3 brothers and 4 sisters.