The 26-year-old songtress coordinated the "Blank Space" performance with Max's sister Ali, reported E! online.
"So Taylor swift showed up to my cousins wedding... #Surprised," Gage Simmons captioned a video of the singer.
In April, Ali reached out to the "Shake It Off" crooner, about the death of her and Max's mother in a touching letter, telling Swift how her brother had wed his wife in the hospital so their mom wouldn't miss out on one of the biggest days of her son's life.
Swift also made a special card for the married couple, writing "So it's gonna be forever" in watercolors with a floral circle.