What your sister is doing is rude. It's immature. It shows a lack of professionalism. You may be holding her in higher esteem than she deserves. With that level of inappropriate behavior, I doubt she is all that great in her job.

No matter what field a person is in, what they know is small compared to what they don't know. Mature, intelligent people are the quickest to acknowledge that. People with the highest credentials are the quickest to acknowledge that. Einstein used to say that he wished he wasn't so bad at Math. (He was hardly any slouch dealing with numbers.) Your sister is all taken with herself and her insights. She lacks insight into herself. If she had more, she wouldn't need someone to tell her that she ought to shut up.

People who think they've figured out everything about everyone around them, probably have a lot of unfinished work to do on themselves. No wonder she's defensive.

Make up your mind to not enable this tendency of hers. It's very loving of you to be so tolerant. But look how it's getting on your nerves. Try interupting her when she goes into analysis mode. Tell her this is a social setting and that she needs to leave her work hat at work.