Find how much pizza is left with John.
1/6
Amount of pizza eaten by John = 2/3
Amount of pizza given to his sister = 1/6
So, total pizza eaten by John and his sister = 2/3 + 1/6
= (12 + 3)/(18)
= 15/18 = 5/6
Therefore, Amount of pizza left with john = 1 - 5/6
= (6/6) - (5/6) = 1/6