Night of the Comet

The Earth is passing through the tail of a comet, an event which has not occurred in 65 million years. The last time coinciding with the extinction event that wiped out the dinosaurs. On the night of the comet's passage, which takes place eleven days before Christmas, large crowds gather outside to watch and celebrate.
18-year-old Regina "Reggie" Belmont works at a movie theater in southern California. She is annoyed to find the initials DMK have the sixth highest score on the theater's arcade game on which all the other scores are hers. She stays after the theater closes to play until DMK's score is removed and have sex with her boyfriend Larry, the theater projectionist, in the steel-lined projection booth. Meanwhile, Reggie's 16-year-old sister Samantha "Sam" argues with their stepmother Doris, who punches her in the face. Sam then spends the night in a steel backyard shed.
The next morning, a reddish haze covers everything, and there are no signs of life, only piles of red dust and heaps of clothing. Unaware that anything strange has happened, Larry goes outside and is killed by a zombie wielding a pipe wrench. When Reggie goes looking for Larry, she encounters the zombie, but escapes. She heads home to find her sister. Because both Reggie and Sam spent the night shielded from cosmic effects by steel, they were saved from the comet's effects.[3]
After figuring out what has happened, they hear a disc jockey and race to the radio station, only to find it was just a recording. They come across another survivor there, Hector Gomez, who spent the night in the back of his steel truck. When Sam talks into the microphone, she is heard by researchers in an underground installation out in the desert. As they listen to Reggie, Sam and Hector debate what to do, the scientists note that the zombies, though less exposed to the comet, will eventually disintegrate into dust themselves. Hector leaves to see if any of his family survived, but promises to come back. Reggie and Sam then go shopping at a mall. After a firefight with some zombie stock boys, the girls are taken prisoner, but are saved by a rescue team sent by the scientists.
Reggie is taken back to their base. Audrey White, a disillusioned scientist, offers to dispose of Sam, who was diagnosed as having been exposed, and to wait for Hector. After she fakes euthanizing Sam by injecting her with a sedative that only put her to sleep, she kills the other remaining scientist. When Hector returns after an encounter with a zombie child, Audrey briefs him on the situation and then gives herself a lethal injection. He and Sam set out to rescue Reggie. The researchers had suspected and prepared for the comet's effects, but inadvertently left their ventilation system open and the fans running during the comet's passage, allowing the deadly dust to permeate their base. Meanwhile, Reggie has become suspicious, escapes, and discovers that the dying scientists have hunted down and rendered healthy survivors brain dead. They harvest their untainted blood to keep the disease at bay while they search desperately for a cure. Reggie saves a boy and a girl before they are processed, then unplugs the other victims from their life support machines. Hector and Sam get the trio out of the base.
Eventually, rain washes away the red dust, leaving the world in a pristine condition. After Reggie pairs up with Hector, Sam feels left out, as the others are kids. When she ignores Reggie's warning and crosses a deserted street against the still-operating signal light, she is almost run over by a sports car driven by Danny Mason Keener, a teenager about her own age. After apologizing, he invites her to go for a ride. As they drive off, the car is shown sporting the initials "DMK" on the vanity plate.

Question: How are Sam and Reggie related?
Answer: Sisters