This could be due to many factors like your sister may be undergoing some stress or depression. Nothing is much clear from your question. You are making speculations which require more cross-questioning from a professional side. Like how frequent are these unfunny lying incidents, what she says if inquired about her "lies", how are her emotional reactions to arguments or disagreements, what more she says about her husband or in-laws or marriage, and how have you established the lie as a lie. Etc. Etc. Plz consult a psychologist to know more or better gather some more information about her behaviors and emotional state.