It's the 2nd. Right?
Farthest relative vs furthest relative
It depends on what you mean. If you mean a relative that lives far away, I suppose it could be either of the two. If you mean it in the sense of a relative who is distantly related to you, it would be the second.
It depends on your meaning and your dialect. Some dialects make the distinction with "farthest" for the sense of "seperated by the greatest physical distance" and "furthest" for other senses. Other dialects (e.g. AusE) don't use "farthest".