The things you mention are based on her misuse of information you offered her.
So why not just stop offering information?
Just because someone asks you a question, that doesn't obligate you to answer. You can still be civil at family gatherings and cooperate in the care of your family, including your nephew, without setting yourself up for misery. I'd just block her on social media and no longer confide anything beyond the weather, but I wouldn't dramatize a big cutoff and position myself as her victim. That might feel great at the moment, but it won't buy you one grain of serenity when this role conflicts with your obligations and opportunities to engage the rest of your family when sis will be present.