I'm sorry your sister disappointed you like that. I don't see how anger on her part was in any way called for. It might be safe to guess that your sister doesn't like your girlfriend. Even that's no excuse. I don't particularly warm up to my brother-in-law, but I'ld never speak negative about him to my sister. Being with him makes her happy, so I accept them as a package deal. Has your sister always been real opinionated about who you have in your life? This has got to be part of some larger pattern.

I don't know is understanding your sister's attitude would make it easier for you to deal with her, but you might try to analyze where she's coming from. There's probably clues. How well are her relationships going? I'll bet she is quite unhappy about something in her own life.

Unexpected negativity, like you just got from your sister can upset your mental equilibrium. Maybe it would be best to distance yourself from her for a bit. Also, give some thought to the boundary issue. Your sister seems quick to think she knows what's best for you. She pops off with unsolicited judgement, which may reflect that you've been too open with her. She is wanting to get into your business because maybe she's been getting away with that for a while. We like to think that a sister should be a good person to confide in. That depends on the sister. You might want to set more of a boundary between your sister and you.