"They'll get justice. I believe justice will be served, and they will get what's coming to them," Ashley Markham told CBS' "The Early Show" Wednesday.
Seven suspects are already in custody for the killings and police are, reports CBS News correspondent Terrell Brown.
The wealthy Florida couple had 13 adopted children, many of them with special needs. Nine children were home at the time of the fatal attack, though none were hurt.
Markham, one of four adult children from previous marriages, said the Billings "had one mission in life. And that was to give these children joy and happiness. To provide them with lives that they have otherwise would not have known. And we planned to try to continue that. They loved their children. That was what their world revolved around."
Not all of the children have been told the details of what happened to their parents.
"We're trying to keep [the children] sheltered as much as possible until we can get them all sat down and explain to them in detail what is going on," Markham told "Early Show" co-anchor Maggie Rodriguez.
Markham praised police efforts to round up the suspects, but acknowledged that there was still a "long road" ahead in search of justice.
"But I have faith [the authorities will] be with us and find the answers for us."