Explanation:
Draw a circle and split it into thirds. The sister will take one of the slices, so we have 2 left over. That accounts for the 2/3 left over.
Then split each of those two slices in half to get 2*2 = 4 smaller slices. The mother takes 3 of those smaller slices which represents 1/2.
Or you could say (2/3)*(3/4) = (2*3)/(3*4) = 2/4 = 1/2.
The diagram is shown below. The white portion is the 1/3 the sister gets at first. The pink portion represents the fractional amount the mother drank, which is 3/6 = 1/2.