It all comes down to Slaine underestimating how bitter she is about all this. We've seen that Slaine doesn't read/care about her enough to know how much she misses him, and at the same time feels bad for her.

I'm not expecting anything grand, mostly a sudden knife-in-the-abdomen move.

edit: yeah... that would suit her. She'd even find it romantic and satisfying to finally "take Slaine from her sister" before committing suicide.