Suppose a family has three children (consider all combinations -- there are eight of them), and a child picked at random from all children of such families. If he is a boy, what is the probability that he has two sisters?

In this case, I did the following:

P(Boy random | Two sisters in a family) = P(Two sisters in a fam. | Boy random)* P (Boy random) / P (Two sisters in a family). (Bayes Theorem.)

The answer is supposed to be 1/4. However, I got 1/3. Do you guys see why that is?