Does it matter whether we say 'αδελφή/αδελφός' or 'αδερφή/αδερφός'?
No; either is fine for "sister/brother".
"αδερφή" didn't accept.
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.
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.
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
I deleted my previous comment.
"He's got a sister" why not?
Yes, it's fine. We had "he has got..." but not "He's got..." Now, it's been added.