I'm Not Ready for Christmas (2015) on Netflix

Plot summary

Holly Nolan works as an advertising executive, and lying has become commonplace for her. Her ten-year-old niece Anna has a solo in a Christmas recital, singing We Wish You a Merry Christmas, and waits for Holly to arrive. Holly misses the recital and gives Anna a fib to explain her absence. Anna carries her disappointment to Santa Claus and wishes that her aunt be made incapable of telling a lie. The wish comes true, and Holly can no longer lie. With Christmas coming, her career and her love life fall into disarray, and she has to work out how to tell the truth and still do what is right.