P(twice boy|two boys) = 1
P(twice boy|(older girl and younger boy)) = 0.25
P(twice boy|(older boy and younger girl)) = 0.25
P(twice boy|two girls) = 0
So P(two boys|twice boys) = P(two boys) * P(twice boy|two boys)/(0.25*( P(twice boy|two boys) + P(twice boy|(older girl and younger boy)) + P(twice boy|(older boy and younger girl)))) = 1/3.