The eight victims, who ranged in age from 16 to 44, were found dead in four separate homes on their farm in Pike County in 2016.
Parents George “Billy” Wagner III, 47, and Angela Wagner, 48, were arrested alongside their two sons George Wagner IV, 27, and Edward “Jake” Wagner, 26.
They were charged with “planning and carrying out” the murders, along with a “slew” of other offences including conspiracy, engaging in a pattern of corrupt activity, tampering with evidence, and obstruction of justice, Ohio State Attorney General Mike De Wine said.
Jake Wagner was a former boyfriend of one of the victims, 19-year-old Hanna Rhoden and shared custody of their daughter at the time of the killings.
The 19-year-old was in bed next to her four-day-old when she was killed. The newborn survived, as did a six-month-old and a three-year-old also on the farm.
The attorney general said in a tweet: “Our investigation alleges that the suspects studied the victims’ habits and routines. They knew the layouts of their homes. And they knew where they slept.”
In separate comments, he said the case involved an “obsession with the custody and control of children”.
The Pike County Medical Examiner said every victim died from gunshot wounds, with one victim shot nine times.
The victims had soft tissue bruising, indicating a struggle, according to the medical examiner.
Officials said they discovered marijuana “growing operations” at three of the four crime scenes.
Mr De Wine described the murders at the time as as a “pre-planned execution”. Authorities have yet to give full explanation of what they believe was the motive for the killings.
The state attorney general’s office called for information about the Mr Wagners in 2017, asking for members of the public tell them about the family’ vehicles and guns.
The request prompted the Wagners to complain they were being “harassed while the real killer or killers are out there”.
Both Jake Wagner and Angela Wagner told the Cincinnati Enquirer at the time they were not involved in the deaths.
Following the four arrests John Clark, a lawyer for the Wagners, said: “We look forward to the day when the true culprits will be discovered and brought to justice for this terrible tragedy. The Wagners are also very hopeful that in the ensuing months there will be a thorough vetting of all the facts.”
The victims were 44-year-old Kenneth Rhoden, Christopher Rhoden Sr, 40, Dana Rhoden, 37, Clarence “Frankie” Rhoden, 20, Christopher Rhoden Jr, 16, Hanna May Rhoden, 19, Hannah Gilley, 20, and Gary Rhoden, 38.
The Wagner family lived in Peebles, Ohio, at the time of the killings, but they have since moved to Alaska.
The mothers of Billy Wagner and Angela Wagner, Fredericka Wagner and Rita Newcomb were also arrested in connection with the alleged cover-up of the massacre, the attorney general also said on Tuesday.