We let x as the height of Myah
y as the height of ally
z as the height of the brother
x = 4ft 2in = 50 inches
y = x + 10 inches
z = y / 2
solving the equations,
y = 50 + 10 = 60 inches
z = 60 / 2 = 30 inches = 2 ft 6 inches tall
Hope this answers the question.