(NEW YORK) -- Six-year-old Arielle Pozner was in a classroom at Sandy Hook Elementary School when Adam Lanza burst into the Newtown, Conn., school with his rifle and handguns. Her twin brother, Noah, was in a classroom down the hall.
Noah Pozner was killed by Lanza, along with 19 other children at the school, and six adults. Arielle and other students' siblings survived.
"That's going to be incredibly difficult to cope with," said Dr. Jamie Howard, a clinical psychologist at the Child Mind Institute in New York. "It is not something we expect her to cope with today and be OK with tomorrow."
As the community of Newtown begins to bury the young victims of the school shooting on Monday, the equally young siblings of those killed will only be starting to comprehend what happened to their brothers and sisters.
"Children this young do experience depression in a diagnosable way, they do experience post-traumatic stress disorder. Just because they're young, they don't escape the potential for real suffering," said Rahil Briggs, a child psychologist and professor at Montefiore Medical Center in New York City.
Arielle and other survivor siblings could develop anxiety or other emotional reactions to their siblings' death, including "associative logic," where they associate their own actions with their sibling's death, Howard said.
"This is when two things happen, and (children) infer that one thing caused the other. (Arielle) may be at risk for that type of magical thinking, and that could be where survivor's guilt comes in. She may think she did something, but of course she didn't," Howard said.
Children in families where one sibling has died sometimes struggle as their parents are overwhelmed by grief, Howard noted. When that death is traumatic, adults and children sometimes choose not to think about the person or the event to avoid pain.
"With traumatic grief, it's really important to talk about and think about the children that died, not to avoid talking and thinking about them because that interferes with the grieving process," Howard said.
Children may also have difficulty understanding why their deceased brother or sister is receiving so much -- or so little -- attention, according Briggs.
"I think one of the most challenging questions we can be faced with as parents is how to 'appropriately' remember a child that is gone. So much that can go wrong with that," Briggs said. "You have the child who is fortunate enough to escape, who thinks 'Why me? Why did my brother go?' But if you don't remember the sibling enough the child says 'it seems like we've forgotten my brother.'"
"They may even find themselves feeling jealous of all the attention the sibling seems to be receiving," Briggs said.
Parents and other adults in the family's support system need to be on alert, watching the child's behavior, she said. Children could show signs of withdrawing, or seeming spacy or in a daze. They could also seem jumpy or have difficulty concentrating in the wake of a traumatic event.
It will be vital in the next weeks for parents of surviving siblings to return the surviving child to a normal routine, including regular meals, sleeping and physical habits, Howard and Briggs said.
If a child's appetite or sleeping habits change, or if they show any regressive behaviors, including wetting the bed or trouble separating from their parents, it may time to seek professional help, experts said.
Copyright 2012 ABC News Radio