That's just a simultaneous equation.
Let g be the age of the girl.
Let b be the age of the brother.
Let f be the age of the father.
G = 2b = 0.5f
B+22 = (f+22)/2
2b + 44 = f + 22 2b + 22 = f
G = (2b + 22) / 2 = b + 11
2b = b + 11
B = 11
G = 2b = 2*11 = 22