Most people kind of disappear when they get into relationships. I've had a lot of friends that just aren't around when they aren't single. However, your sister's boyfriend sounds a bit over the top with everything. It sounds like she's head-over-heels in love with him, which means she probably won't be able to examine the situation and evaluate your advice effectively. From my past experiences with this kind of thing, there really isn't much you can do except to stick by her.
Show her that you still love her, even if you don't like her boyfriend. Voice your opinions of him if you must, but this will likely just push her away further. I would say something like... "I really don't think your boyfriend is good for you, and I don't think you're going to be happy with him, but I don't want him to come between us" And leave it at that. Eventually something will happen and his spell over her will be broken! When that happens, she'll really need you to be there for her, even if you feel hurt now.
Show her that you still love her, even if you don't like her boyfriend. Voice your opinions of him if you must, but this will likely just push her away further. I would say something like... "I really don't think your boyfriend is good for you, and I don't think you're going to be happy with him, but I don't want him to come between us" And leave it at that. Eventually something will happen and his spell over her will be broken! When that happens, she'll really need you to be there for her, even if you feel hurt now.