Kamillions (1990)

Nathaniel Pickman Wingate has opened a gateway to another dimension using equations and equipment in his basement laboratory. His wife, Nancy, wants him to get ready for his own birthday party. He wants his son Sam to help him. Sam is up in his room looking at pictures of Jasmine on his computer, and a poster of her arrives which he puts in his closet. Although it is Nathan's birthday, the family is enthralled by a visit from Cousin Desmon, who is now a count in Liechtenstein. While Sam is away getting equipment for his father with his friend Alex, his father gets sucked into the other dimension, and a creature from the parallel universe escapes, pursued by another. The first temporarily traps the second with its spit, attacks Desmon, and becomes a duplicate, absorbing his thoughts from the unconscious body. The other manages to get free, and unable to find a human to mimic, finds the poster of Jasmine, and becomes her. Sam soon finds her, and becomes his new girlfriend, but she has to find the false Desmon and take him back to their dimension to keep him from harming himself or Sam's family. Sam's sister Linda ("Lindy") is obsessed with talking on the phone, and can neither spell "Desmon" (she spells it "Dezmon") or Chameleon (she spells it "Kamillion", hence the title), what Nathan called the creatures on his tape recorder. The false Desmon drives away Emma the French maid, and plays childish, seemingly deadly pranks on the rest of the family: Angelica, a slut who owns half the house, Larry, a minister, supposedly born again, but badly sinning, and Floyd, a mechanic. Both chameleons try to figure out the ways of the new dimension, while Desmon finds new ways to make mischief, but I can't give away why. Meanwhile, everyone is operating on Nathan's belief that after the four hours of coolant runs out, half the planet will be blown away.