I was leader and had two players come and tell me about a fellow family member talking about killing me (just because I was leader and she wanted to be). Later I heard the woman in question talking about it and exiled her. Later, she threatened me with a knife and other players were getting annoyed/upset with her, so I tried to kill her but found I needed a posse of 8 - yet my brother was able to kill her.

What exactly was going on in this situation? What does it mean to exile exactly? Do I need to say "I will kill..." before others can "join" me? Do other players have a lower posse requirement than the leader? Would this player have been able to kill me?

I'd just like to not fumble through such a situation again as a leader.