He admitted he stole the car he crashed from his uncle's home.
Washington County District Attorney Kevin Kortright said he opted to drop the murder counts in light of concerns over whether his office could prove St. Andrews acted with "depraved indifference toward human life" before the crash.
Kortright also said he didn't want the victims' families to have to go through a trial.
The accident led to St. Andrews' parents being convicted of crimes this spring by a Washington County jury in Fort Edward.
Jimmy Joe St. Andrews and Kellie St. Andrews of Putnam were also found not guilty of other charges.
The jury found Jimmy Joe St. Andrews, 41, guilty of endangering the welfare of a child, but not guilty of two other endangering charges.
Kellie St. Andrews, 40, was found guilty of endangering the welfare of a child and unlawfully dealing with a child. She was found not guilty of three other counts of endangering the welfare of a child and three additional dealing with a child charges.
Kellie St. Andrews, 40, and her husband Jimmy Joe St. Andrews, 41, were sentenced to 60 days in jail and three years probation in Washington County Court April 3. They were also fined $1,000 each and must perform 100 hours of community service.
The St. Andrewses were on trial in connection with several under-age drinking parties at their Putnam home, the last of which came last April 20-21. The two 16-year-old girls died in a car crash after that party.