It was the maximum possible sentence for Mark Mullan, 51, who has five previous drunk driving arrests on his record.
Mullan had a blood-alcohol content about three times the legal limit when he hit the family as they crossed a street in the Wedgwood neighborhood in broad daylight March 25.
He was on probation from a recent case, his license was suspended, and he was supposed to have installed an ignition interlock device in his truck.
He pleaded guilty in October to vehicular homicide and vehicular assault in connection with the deadly crash. Prosecutors recommended an 18-year prison term on Friday and Judge Barbara L. Linde agreed,
New grandparents Judy and Dennis Schulte, retired teachers from Kokomo, Ind., who had recently moved to Seattle, were killed in the crash.
Their daughter-in-law, Karina Ulriksen-Schulte, suffered a broken pelvis and head injury, causing a stroke that has limited her speech. She remains in a wheelchair. Her baby son Elias sustained brain injuries and may be blinded for life.
Dan Schulte, the deceased couple's son, said it is "difficult to transcend the horror" of that tragic day.
The case has prompted calls for tougher DUI laws. The family has sued the city, alleging it failed to monitor Mullan.
King County Prosecutor Dan Satterberg noted that the Legislature's decision to toughen vehicular homicide laws in 2012 provided for a longer prison term for Mullan.
Some 150 people a year are killed by drunken drivers in Washington state.