Madame Meiru will command you to find 5 items that belong to Luig's brother Mario. With each item she can channel herself to the spirit world and provide you with information. After you have found all five items and spoken to her, you can attack her for a key. These are the items and where to find
them:

#1 - Mario's Hat - In the washing machine in the laundry room
#2 - Mario's Letter - Near the birdhouse in the backyard
#3 - Mario's Star - In the observatory
#4 - Mario's Glove - In the cinema room
#5 - Mario's Shoe - In the twin's room