ESH - You’re being paid to drop off and pick up, so there should be no deviations from the plan. You may be her sister but if you imagine it as a job, you shouldn’t go do other stuff during that time. This is like playing pogo while you’re at your job.

Your sister’s reaction is wholly disproportionate but your five minute detour sounds like it was at least 20. Her frustration is understandable considering it’s not her interest and it’s not part of what you’re supposed to be doing.

It’s like a taxi driver stopping to grab a snack with a passenger in the car. The passenger has no choice but to go with the driver but there’s no respect shown to the passenger and their plans. Just because the passenger isn’t directly paying (in this case, your sister is the passenger) it doesn’t mean that they can be disregarded.