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The age for restricted license is 14 years 6 months in South Dakota, 15 years in Idaho and Montana, and 15 years 6 months in South Carolina and New Mexico.
Usually restricted licenses are for drivers under 18 and restrictions might be hours of day and age of passangers. I.e. they would formally not allow the twin sister in the game to drive the car after the MC is hit in the head, since it happens late at night. Though I guess a cop might decide it's ok given circumstances
Then there's the hardship license, but those are for kids in hardship situations, for example where they have to drive to school since it's the only way to get there and no-one in the family are available at that time to drive, or if the parents can't drive due to medical issues and the child has to drive for them in all situations. Which is not appliable here either really.
Imho, the main decider of the age of the twins is the fact that the studious one is studying to be a lawyer (iirc?) and has some sort of intership or ride-along with the local cops. Which makes it decidedly improbable that she is younger than 16, and more probable that she goes to college. Which means 17 or 18 yearsold, unless maybe she goes to a weird private college and is a prodigy.