Based on this drawing: https://www.furaffinity.net/view/36707766/

As Loona and Zatana sat in their thick diapers, Zatana noticed a pump nearby. A devilish grin formed on her face as she waddled over to the the pump and headed back over to slip it in Loona's diaper. Not too long after, she started pumping.
Loona put her phone down for a moment to see that her diaper was ballooning. Much to her further surprise, she started to float upwards. "Zatana, stop!" Loona shouted as she waved her hands. "Sorry, had too much fun." Zatana answered before she finally stopped.
Moments later, the pump was put away before Loona was changed. Apparently, she felt much better after that. (even if her sister sometimes drove her nuts)
THE END