From the Super Mario Wiki
Patty's mother and father, after their daughter disappeared, went out to search for her and found her at Mario Brothers Plumbing. They eventually learned from Mario and Luigi that Patty had been staying at their place, claiming to be an orphan without a home.
Patty's parents told Mario and Luigi that Patty was a chronic liar and troublemaker, meaning that she was a "kidder" once telling their neighbors that they were "lost in space". Patty's parents proceeded to ground her "until the end of the century" illegally for tricking Mario and Luigi.
Regardless, Patty and her parents decided to stay at Mario and Luigi's apartment for the party that was originally going to be Patty's birthday party.