When everyone gets held at gunpoint by Pablo Chacon, David, who's been a Jerk Ass for most of the film finally does something selfless and tells Pablo to kill him instead and talks about how great Rose, Casey and Kenny are.
At the end of the film, the Millers become a real family.
David describing a memory of Rose to the Fitzgeralds on the first time he saw her.
When Casey comes back from her date, David and Rose turn her catching them about to kiss back on her. They act like parents upset with their daughter being out past curfew. No one calls anyone out on not being a family, and Casey storms off to her bunk and smiles a little about them caring.
A subtle one, but when Kenny gets bitten by the tarantula, Casey and Rose are just as concerned for his health as an actual mother and sister would be, despite neither one having known him for more than a few days.