A relative, connected either by blood or through marriage, that has a tendency to act out, either in public on online, in less than pleasant manners. Typically, the idea of a family tree limb that is considered "redneck" or "whitetrash," and whose actions typically link them with the less refined, less dignified, and generally unappealing. They repel other family members based on their history or previous experiences.
Oh great. My repelative cousin just posted on my Facebook wall that our grandmother has just died...she didn't even bother to call me and tell me!