Sal and her grandparents are driving to the grave site of Sal's mother. The group is trying to get to the grave on or before the birthday of Sal's mother. At one point during the driving, the threesome make a stop and Sal's grandmother gets bitten by a poisonous snake. From that point forward in the story, the grandmother's health is in decline.
As they get close to Lewiston (the grave location), the grandma goes unconscious. Both Sal and grandpa are very worried, and grandpa decides that they have to get to the hospital in Coeur D'Alene as quickly as possible. The road is windy and mountainous. So Sal is worried about two things, grandpa driving that road while worried about his wife, and Sal is worried about her grandma's health.
That's the part when grandpa is driving. At the hospital, the grandpa will not leave his wife, but still wants Sal to make it to the grave location on time. So what does he do? He gives his car keys to a 13 year old. Sal drives in the middle of the night on a treacherous, winding mountain road. She's worried about her own safety, since she knows that very same road killed a bus load of passengers.