The 21-year-old joked during the concert about keeping an eye on his sister, reported E! online.
"I'm sorry if I seem a little distracted, my sister's on a date and I'm trying to keep an eye on it. They're up there somewhere," he joked.
"So, just please behave yourselves, it's disgusting," he added.
Liam Payne also joined Styles and joked, "We're all watching you!" The concert came shortly before the popular boy band made the announcement of its fifth album, titled "Made in the AM".