Yes, this is a problem (I think from alpha version, and I'm not sure because I'm relatively new as a contributor), but now that we are on beta the tree's locked and we can't change the main sentences (at least most of them, with only a few exceptions), just the translations. It's on your to-fix list though, thank you for your comment. ^.^
It's mandatory for technical reasons, because the absence of double accent causes confusion sometimes. For example:
Ο ξάδερφος τής έδειξε ενα βιβλίο (The cousin showed her a book) - Ο ξάδερφός της έδειξε ένα βιβλίο (Her cousin showed a book).
Two sentences with the same structure, yet a different meaning, depending on where the accent goes. It's not something too hard to get used to though. Plus, if you get used to speaking Greek, or hear someone else speaking, and practice it long enough, accents are going to sound like the most natural thing in the world. ^.^
If you typed ο ξάδερφός μου και εγώ, that should be accepted.
The word should have two accents since it is followed by unstressed μου -- the original sentence (Ο ξάδελφος μου και εγώ) can't be changed in this version of the course at this point but all the additional alternatives use only the correct, two-accent version.
There's no difference in meaning between ξάδερφος and ξάδελφος; it's a bit like "aluminium - aluminum" in English. One spelling is a bit more "historically accurate", the other reflects a pronunciation change in many Greek dialects, and both are in use.