Interesting how he turned the situation around and called you co-dependent when he's the one who caused the problem in the first place. I'm sure that your kitty can sense how he feels about her even though he's now trying to be nice to her. I understand that this is probably a sticky situation for you because it involves family but I would be sure that my cat is nowhere in the perimeter when he's around. If she truly had been abused before she came to you, of course she's going to be erratic in her behavior around others, especially those who have no tolerance for her. Personally, I'd tell my SIL "hands off" and if he didn't like it, oh well.