(NECN: Alysha Palumbo - Boston, MA) - It was story that shocked a community. Two years ago, 39-year-old Marcy Thibault was driving her five-year-old niece and four-year-old nephew to her Bellingham, Massachusetts home for a sleep over, when she stopped on 495 in Lowell and carried the two children out onto the highway, where they were hit and killed by cars. Attorney Andrew Meyers said, "Certainly the family would never have allowed their children to be in her care if they had any concept whatsoever that she could or would have a dangerous event that she had." Thibault had been hospitalized four months earlier for mental illness. But psychiatrists at McLean Hospital discharged her less than a week later. Now Thibault's sister and brother-in-law, Danielle and Ken Lambert, are suing McLean Hospital and three of its staff members for allegedly failing to properly treat Thibault or warn them that she could relapse. As the executive director of the Massachusetts chapter of the National Alliance of Mental Illness, Laurie Martinelli says outwardly patients can appear to be improving when they're not. "It's an illness that goes up and down," said Martinelli, "and because there's a lack of education about it in general and there's a tremendous stigma around mental illness it's really unfair I think to put any kind of owness on family members. Martinelli says confidentiality laws prevent hospitals like McLean from speaking with family members who are not the spouse of an adult patient about their treatment and diagnosis. For its part, a spokesperson for McLean Hospital says the hospital cannot comment on pending litigation. Meyers said, "They believe that had there not been a breakdown of communication or an improper diagnosis and information given to them about Marcy that their children would be alive today and Marcy would be alive today." The Lamberts' attorney, Andrew Meyers, says they hope this lawsuit will expose what he calls, "failures in the mental health system."