The Baldwhen Triplets The Final Showdown

Liz, Steven, and Jeff can't believe they are already graduating from high school. To make things even better their sister finally arrives and a few weeks later so does thier brother. With their brother though comes bad news.
What was the true reason they were sent away from their home planet, and can they stop the certian doom their brother's ship warned them about? And what about their own happy endings?