Stepsister from Planet Weird 2000

Directed by Steve Boyum

Megan's world is turned upside down when she hears her calm life with her little brother and single mum is about to change. She hears she's soon to have a stepfather and a stepsister. Megan thinks they're a bit weird and is determined to stop the wedding. She discovers they're actually even stranger than she thought - they're from another planet.