William your sister is acting like a JW automaton, acting not thinking. I am close to my sister and would hate to be shunned by her so I can see how you must feel.
Nevertheless a JW is stuck in a mind-bending cult and you are not. Perhaps this is the time to tell her simply that your love for her is unconditional and that she should look into the hypocrisy of the JW organisation on the internet.
The result is still likely that you will be sent to Coventry by her but you have the prime opportunity to leave her with a big doubt about her wonderful religion.