I'm also curious if any other elements seem to align. Do you think you could ask your sister any more questions about it or do you think she would rather not?

Did the stabbing happen outside on their front lawn or did she hear a commotion and see them through their windows? Is it possible that it took place in their kitchen?

Did she back off on her own like in the dream or did her boyfriend have to get the knife away from her? Did he scream at her after the stabbing or was he just trying to get away from her?

And, from a different angle, did you find out what she was thinking when this was unfolding? Did it cross her mind, at the time, that she'd feel safer if you were there or could she have been thinking something like "My brother will never believe what just happened!"?

Of course, it doesn't really matter but it would be fun (that seems like the wrong word in the context of "finding out details on a stabbing", but I suspect you'll get what I mean) to find out it's true. I already feel like it's an odd enough coincidence that I can add it to my list of "possible anecdotal evidence that we are be able to, on some level, transmit our thoughts, experiences or emotions to others, especially in times of intense emotions and between people who have a close bond". I feel like a lot of us, if we were asked whether we definitely believe telepathic connections exist would probably say no but at the same time most of us have come across some tales (of "I felt something was wrong the moment my husband got into a car accident on his way to work" or some variation on that theme) that make it difficult to rule out the possibility.

Thanks for sharing, either way.