"We're never going to move on," Jillian Soto said. "My sister got married and [Victoria Soto] wasn't there for her wedding. I'll graduate in a year from school and my sister won't be there. ... You don't move on from your sister being brutally murdered in her first-grade classroom. ... We live every day with the pain that our sister's gone."
On Dec. 14, 2012, Adam Lanza, 20, killed his mother before storming into the Newtown, Connecticut, school and killing 20 children and six adult school employees. Five of the 16 kids in Soto's class were slain.
The families of 10 victims filed the lawsuit Friday against the maker, distributor and seller of the rifle. In addition to Bushmaster, the families named Camfour, a firearm distributor, and Riverview Gun Sales, the store where the Bushmaster rifle was purchased in 2010. The lawsuit alleges negligence and wrongful death.
"I can't say if my sister would be alive if Adam [Lanza] walked in with other types of guns because no one knows what could have happened but Adam walked in with that gun that day, into my sister's classroom and killed my sister along with 25 other people," Jillian Soto told ABC News.
The Sotos said the families had decided that the next best route would be going after the maker of the gun and the people who had sold it.
"I hope to get the gun industry to accept responsibility," Jillian Soto said.
Bushmaster did not respond to ABC News requests for comment.
Under federal law, however, gun manufacturers have blanket civil immunity from liability resulting from the criminal or unlawful misuse of their weapons. This applies in federal and state courts.
The 2005 law, the Protection of Lawful Commerce in Arms Act, effectively shields manufacturers and dealers when someone uses a gun in commission of a crime. There are some exceptions in the law.
One of the exceptions is for "negligent entrustment," which allows lawsuits to proceed if it can be shown that a seller was negligent in allowing a person to possess a gun "when the seller knows, or reasonably should know" the person is likely to (and, in fact, does) use the gun to harm himself or others.
The Newtown families are asking a state judge to broadly apply that negligent entrustment exception.
They are not alleging negligence against the seller for specifically allowing Lanza to possess the weapon -- it was purchased legally by his mother -- but rather that all the defendants are negligent for marketing and selling what is essentially a military weapon for civilian use, and for entrusting any civilian to possess one.
"If you’re designing a perfect hunting rifle you wouldn’t design this. They took a product -- they took a military weapon -- and they wanted to know how to make money off of it, and you know you can’t begrudge them for trying to make profit out of a weapon but at what point do we, do they realize that there are Adam Lanzas out there right now who can get their hands on these weapons?" asked Joshua D. Koskoff, an attorney for the families in the lawsuit.
Jillian and Carlos Soto told ABC News that it was still "hard to grasp" that their sister was not alive. They said their family wanted to keep her memory alive and help to prevent another tragedy.
"We wanted people to remember that she was a fighter and we're gonna fight for change in the gun industry," Jillian Soto said. "If this lawsuit doesn't pass, we're going to continue to fight for gun change. We will continue to tell our story. ... Change needs to happen, one way or another."
ABC News' Tom Llamas, James Hill and Enjoli Francis contributed to this story.