The unforgivable sin I would say is that dangerous ignorance decreed by the GB that obedience to them is more important than life. Jehovah's Witnesses have re-introduced what effectively is human sacrifice through their no-blood teaching.
How many suicides have there been among JWs because of their doctrines, especially on homosexuality?
Do they care about the welfare of the flock? Not at all! As the Royal Commission in Australia found out, they are only interested in their good name...forget the emotional needs of the abused.
This is what sin is.