"I rang my sister and told her that I no longer believe JWs have the truth and the phone went silent, when I asked her to say something she just said if that was the case then she would have to hang up on me and then proceeded to indeed hang up."
I think her response highlights the ugly truth about the JW religion. When you really get down to it, it's really not about worshiping and serving Jehovah. It's about the pride of being special, the grandiose thought and religious prestige of being the only true religion.
So when you say that you no longer believe that JWs have the truth, that's a hard blow to their narcissistic religious pride. It pains them more to hear you say that than if you had said that you no longer believe in God. That's why they hate defectors from their religion more than they hate atheists. Defectors assail their true god - pride of religion.
The JW religion is a tribal one and that's why JWs support their tribe (the organization) at all cost. They will lie and they will deny facts in a bid to be loyal, die-hard supporters of the tribe. It's just like die-hard supporters of a sports team. Supporting the team is more important than the game itself. They will hate a team that beats them fairly and make all kinds of excuses and rationalizations for why their team lost. In extreme cases they even attack supporters of the winning team. Tell such a die-hard fan that you don't like the game of soccer. But don't tell him that his team sucks. It's the same with JWs.