Please refer to the article. The 684cM of shared DNA precisely complies with the Group C or with the Group D with a little skew. So it is very probable that your guess is correct. But to prove it, of course, you need more tests, because single test can't distinguish who is uncle and niece/nephew, for example. It doesn't give clues about age or generation of person. Just only the distance between two persons to most common ancestor.

If you are interested please write more information: what is the sex of tested persons (I believe that you are man) and from whom you could get additional tests. Also it is very interesting if there are Y-DNA haplogroup and mtDNA haplogroup data in your and this person's test. It may give some additional ideas about relationship.

Please refer to the article The Limits of Predicting Relationships Using DNA. The 684cM of shared DNA precisely complies with the Group C or with the Group D with a little skew. So it is very probable that your guess is correct. But to prove it, of course, you need more tests, because single test can't distinguish who is uncle and niece/nephew, for example. It doesn't give clues about age or generation of person. Just only the distance between two persons to most common ancestor.

If you are interested please write more information: what is the sex of tested persons (I believe that you are man) and from whom you could get additional tests. Also it is very interesting if there are Y-DNA haplogroup and mtDNA haplogroup data in your and this person's test. It may give some additional ideas about relationship.