Is'nt it weird that the person to replace Kate, my she rest in peace, was her killers half sister?=)

True, that is ironic.
But in another way of thinking: isn't it fitting that the person to replace Kate was the person who avenged her death by killing her murderer?

I don't think so , because Ziva ended up killing him herself. I think she deserves KAte's seat for killing her own half brother.