Okay, I see that you have had a closeness with your sister (I’m guessing you’re the older?) but since your sister’s behaviour has changed your relationship has understandably suffered. It’s good you have the foundation of closeness and although you’ve had difficulties you’ve apologised and kept communication open as much as possible. You sound like a pretty good sister to me, and although she might not acknowledge this hopefully on some level she’s aware of your support.

It does sound like something has happened with her, but it might not be something that’s obvious. I’d suggest keeping your mind open, not jumping to conclusions, and as much as possible listening to her. There may be clues in what she says.

You sound a good person to have on her side.