As the sister, you don't live with the child. It is probably better if Sis comes on here herself. Vague and could be Asperger's Syndrome...sounds a lot like it. Many kids are from divorced homes and they all behave in different ways. Not all are disturbed.To me sounds classically Aspergers...but there's really so little to go on.
Is your sister worried? She's the one who is the parent. And the father. Tell her about us. It is so very hard to go through a third party.