KLM has free 24h cancellation on any ticket. I have used it before when I mistakenly booked a ticket to DUS instead of DRS for example. That was a one-way from AMS so not touching the US in the slightest.
You sister must have seen the "Time to think" option and thought that's what it's for, but "Time to think" gives you the option to book later for the same price, not to cancel within 24 hours.