Why is my sister trying to wreck my relationship with my fiance?

She is like habitually lien to me and my fiance and she is vindictive and and just becoming an AZZ and when me or my fiance confront each other about the story's she tells us they dont add up and my sister keeps saying my fiance is cheating on me but we have been together for a good 7 years one of those engaged and we have been very faithful to each other and i asked my sister why she was doing this to us and her Response was "i don't agree that you should be getting married and if mom and dad wont stop u i will!!!!" WTF????? why is she trying to destroy my engagement????? this makes no sense at all?!?!?!?!?