I hate my sis-in-law, what should I do?

If it were me I would focus purely on your relationship with your bf. This is the most important thing that matters. Take your own space with him and ignore the Sister-in-Law. Try to show no reaction to her when she is behaving this way otherwise you will be giving her the satisfaction she is looking for. She will know she has rattled your cage and will do it more. Your boyfriend chose you not her. Focus on strengthening the bond between you both. Brothers are notorious for latching on to trouble, making it and enjoying it just for the fun and the devilment of it. This being so, ignore him too and don't show any reaction. Good luck and best wishes. uk