How do I keep from cussing out my sister-in-law?

She lies more than any person I've ever known, save one, and I don't know how much more I can take. Her family has just let this go on for so long it has really just become something she does rather than a problem that something needs to be done about. And please don't tell me to just overlook it or it's not hurting anyone or it's just a lie...who's it hurting? If I could do any of those or felt that way....I wouldn't be asking this question. It's just that she will lie about the dumbest crap on earth and just expect for everyone else around her to be just as dumb as she is and believe her. The sad thing is she's a pretty nice person she is just so fake and lies so often I really can't see us becoming close friends like I initially wanted and expected to become. How can I tell her off, and let her know, when I know she is lying without causing some huge family crisis. If this was my immediate family and not my in-laws I would just tell them off.