"I was just sick, just sick to my stomach [over] the fact that she can be free, and my cousin, he's not here," Makenna O’Donnell said in an exclusive interview today on ABC News' "Good Morning America."
"She should be behind bars," O'Donnell said.
Carter was 17 when Roy, her 18-year-old boyfriend, died in July 2014.
Carter was found guilty by Moniz in June for behavior he described as "reckless." In announcing the verdict, the judge said Carter instructed Roy "to get back into the truck well knowing of all of the feelings he [had] exchanged with her, his ambiguities, his fears, his concerns."
"I just don't understand how someone can be free knowing that she deliberately told him to get back in the car and she gets to sleep in her own bed tonight. She gets to eat breakfast; she gets to wake up with her family," she said. "Meanwhile, where's Conrad?"
O'Donnell also said she believes Carter "needs help."
"No normal human being who doesn't have problems would tell someone to get back in a truck where it is a toxic environment," O'Donnell said. "I do believe she needs help and I do believe she needs to take responsibility for her actions."
O'Donnell recalled her late cousin as "funny," "goofy," "selfless," and someone who "had so much life."
"His energy was just unbelievable, out of this world, and the world no longer has that," she said.
"There's always an open door. There's so much more to live for," she said. "There's always a light at the end of the tunnel."
ABC News' Joseph Diaz, Doug Lantz and Emily Shapiro contributed to this report.