250000005
Hint There are 4 ways to choose the first girl. If you choose the first girl of trick, then the trick is uniquely determined. One way is to choose his sister first, and others are to choose 1, 2, 3. If he chooses his sister first, the trick ends in one turn. If he chooses 1 first, then he must follow in order of 1, 2, 3. Then he will notice that they told him a lie and directly go to his sister. So the trick ends in four turns following the sequence of {1, 2, 3, 0}. In case of choosing 2 or 3 first is very similar ({2, 3, 1, 0}, {3, 1, 2, 0}). Each case occurs with the same probability of 1 / 4. So the answer is (1 + 4 + 4 + 4) * 4^(-1) mod (10^9+7) = 250000005