I mean after you find the first guy from the junior class, you still need to find his brother/sister in the senior class, and you have to look for this person in that 60 pair siblings sample don't you?
P(finding sibling pair) = P(finding one of the 60 Jr among 1000)*P(finding his/her Senior sibling)
Alternatively, you could think in terms of number of combinations:
Total combinations = 1000*800
(J1,S1)(J2,S2)......(J60,S60).... 60 such pairs can be found.
P(finding sibling pair) = 60/(1000*800)
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