In The Origami King, King Olly’s sister is named Olivia.
Olivia is introduced as a benevolent origami character imprisoned by her brother, King Olly. Mario rescues her early in the adventure, and she becomes his principal companion as they travel through the Mushroom Kingdom to remove the five colored streamers covering Peach’s Castle.
Olivia’s importance goes beyond being Olly’s sibling. Olly created her using the same forbidden Fold of Life technique that brought him into existence, but she rejects his plan to transform the kingdom and turn its Toads into blank paper. Her compassion and opposition to Olly drive much of the game’s emotional story.
At the climax, Mario, Olivia, and Bowser defeat Olly. Olly reveals that he folded 999 paper cranes and asks Olivia to fold him into the thousandth. She grants his final wish, then uses the resulting wish to undo the origami transformations—including her own existence.
Timpani is a character from Super Paper Mario, while Kersti belongs to Sticker Star. Neither is Olly’s sister; the canonical answer is Olivia.