how Sin is suddenly in the top-tier of the harem with Micah and Nathaniel in this book. Why is that?

I would seriously bet that Sin is now top-tier just to spite Asher for hurting him and for the fact that's he's apparently jealous of him (though Anita made that observation, so it could be totally wrong, but still, if it's in LKH's head, I wouldn't put it past her to try to spite Asher by shoving it in his face that she was staying with Sin and not him >_>)