Yes, it's fine. We had "he has got..." but not "He's got..." Now, it's been added.
The "correct" answer given is "He's a sister.", which makes no sense. Gender confusion anyone?
No, just Duo automatically contracting he has to he's whether that makes sense (he has done it = he's done it) or not (he has a sister = he's a sister).
In a listening exercise, you have to write what you hear.
If the voice says αδελφή, you have to type αδελφή (even if you might yourself use αδερφή) -- and vice versa.
Thank you @mizinamo I do that all the time. I forget to consider the listening exercises and worry that there is a glitch in the computer system.
I deleted my previous comment.
It could make a remark or a suggestion, not to make the all answer wrong only for this difference, which in fact is not even a difference, is right.
The exercises are graded by the computer program. But I might also point out that as this was a listening exercise there is a difference in the pronunciation of αδελφή and αδερφή.