I came from a large family myself. We took family vacations in a station wagon. My kid brother, maybe 6 years old at the time, was asleep when we stopped for gas once and, after everyone had their potty break, we all piled back in the car and were about 15 minutes down the road before I noticed him missing.
Dad did a U Turn and drove back in a panic. He was sitting in the gas station lobby happily slurping an orange soda someone had boughten for him. We asked if he was worried and he replied "No, cuz I knewed you'd come back."
Is your kid bro still a character?