For all of those that requested the word relations to be added as an alternative, I'm afraid that it cannot be accepted.
Relations is indeed used instead of relatives sometimes, in some regions, but it's not commonly seen or heard enough to be added. Even if it was, relations is considered to be a wider term than relatives. Relatives is usually (if not exclusively) referring to family, which is exactly what the word συγγενείς describes. Relations as a term could refer to literally any kind of relationship.
I doubt that the word relative is hard to use for most learners, as most of have probably heard or seen it. We would suggest that you stick with the suggested alternatives, just so there is no confusion, even outside of Duo. ^.^
Hi Dimitra - I wouldn't suggest including it as a main translation, it's just that for me as an English English speaker, I was baffled to have it marked wrong, because relations is in this sense (it has other senses) is entirely synonymous with relatives. And your relations are definitely NOT people you have some sort of non-familial relationship with - they are quite specifically the people your are related to.
In English the word relatives (οι συγγενείς) is considered by some straight-laced persons to be rather non-U. c.f. Nancy Mitford's book: Noblesse oblige. The word relations is to be preferred. I do not know the American version but as G B Shaw said, it looks like another case of : two countries separated by a common language.