How is it possible to have an unexplainable hatred towards a family member?

It sounds like you’re a recent observer to this particular situation or you’re the “hated” family member.

If you’re in the first situation, you don’t have all the information. Therefore, you don’t know the reasons behind the hatred. Don’t involve yourself in the “solution” and don’t try to make everyone get along for the sake of appearances or because conflict makes you nervous.

If you’re in the second situation, trust me, there’s always a reason. It probably won’t be a satisfying one to the person receiving the hatred, however.

Maybe the person hating other people is a mostly hateful individual (a bigot, a misanthrope, etc.). Good riddance to bad rubbish, I say.

However, there’s another, more sinister explanation. Maybe the person accusing the other person of “unreasonably” hating them is intentionally trying to make the “hating” family member look “crazy,” especially to outsiders. Maybe the hated one is doing this intentionally to cover up their own transgressions and abuse of the person that “inexplicably” hates for “no reason.”

That’s gaslighting and it’s a form of abuse in itself. Many abusive family members love to make outsiders think the “black sheep” is irredeemable, flawed, faulty, unfriendly, and a host of other unflattering conclusions. In my experience, the black sheep is usually the barometer of emotional health, honesty, and integrity within families. Nothing like having a scapegoat to cover up for your own failings.