divide 7 people into 2 groups: Group 1 including 4 people have exactly 1 sibling and Group 2 including 3 people have exactly 2 siblings

First person:

If he is from G1 (probability: 4/7), the probability that the second one is not his sibling: 5/6 so 4/7*5/6=10/21

If he is from G2 (probability: 3/7), the probability that the second one is not his sibling: 4/6 so 3/7*4/6=6/21

10/21+6/21=16/21

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