I think that Casey's sister and brother in law tolerate Kyle for the sake of Casey, but really are growing tired of him. Those vlogs with Kevin, you can see that Kevin is tiring of Kyle's behavior , they probably figure that this may run its course and Casey will come to her senses. As for Casey crying in the ride back to the airport, well the poor girl is looking as this as a possible future . I really hope she comes to her senses.I would be crying too if I was stuck in a car with that smelly swamp rat.
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Maybe Older sister feels sorry for Casey about the mother dying? Takes pity on her and makes Kevin participate?