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