They're her biggest fans.
Redditor foundyourlink's little sister has been a fan of One Direction for a few years (she's 15 now), and when she was younger her parents helped her set up a wall display of black-and-white photos of the band made to look like Polaroids. A few years later, they had a hilarious idea and immediately acted on it, a clear indication that they're the awesomest parents ever.
Over the course of a few days, they recreated some of the photos with themselves posing as the band members. While their daughter was at work, they snuck into her bedroom and replaced the photos, thinking it would take her a while to notice.
Instead, she noticed as soon as she got home, and "came running downstairs, crying with laughter to the point where she was drooling. It took my parents longer to put it all up compared to how long it took her to notice them."
"They love their pranks!" says foundyourlink.
Here's how they did it:
(by Shira Rachel Danan)