Unfortunately, your sister probably won't listen, but you could try asking her if the "Faithful Slave" meant it when it said in the July, 2009 Awake! "...no one should have to choose between their family and their beliefs". If they didn't really mean it, or try to claim that it doesn't apply to JWs because "we have the truth," then they have institutionalized hypocrisy. Either that article meant what it said, or it didn't. They can't have it both ways.
None of Jesus' brothers or sisters followed him until after his death. There is no record that he ever shunned them. On the contrary, there is every reason to believe that he treated them the same as anyone else with good sense would treat his siblings.
The only other organization I know of that says once you're in you can never leave is the Mafia. The JWs are the biggest whiners of all when relatives of newly interested ones try to persuade them to stop their studies. They're the first ones to cry "Persecution!" and make a lot of noise about freedom of religion. But if a Witness should do independent research and decide they don't want to be JW anymore, that's a whole different story. There's a BIG price to be paid. Like I said, they're hypocrites.
You don't have to make your mom choose. Just let her know that, come what may, you will always be there for her. Unconditional love means just that... unconditional.
My best to you and yours,
Radar