They should've used 2 different actresses.
Malin Ackerman was the right age to play MAX's mom, but she was too old to play NANCY. Even if all the camp counselors had been the same age, it still would've been too distracting to see a woman not age in 30 years.
DUNCAN's age is never mentioned and there's no reason to believe he's the same age as his sister and her friends, but it felt like he was in the same age group (not more than 5 years older). Therefore, I also think Thomas Middledich was too old to play him.