This game is titled Petz: Hamsterz Life 2 in North America and Hamsterz 2 in Europe.
After you visit your cousin Mary a large number of times you should get an event when your Aunt visits Mary. It starts out like any normal visit, but then after you talk another message appears: <knock> <knock>, and it is your Aunt Sofia. After meeting each other Aunt Sofia is added to your destinations map.
To perform this trick you must have unlocked the birthday cake from the store and have enough stars to purchase it. You will also need to purchase a new hamster of any breed/type. Place your new hamster in the cage of your choice and go to the "Special Items" menu. Select the birthday cake and place it somewhere in the cage away from the hamster. Once you place the cake in the cage you will automatically get over 1,000 stars at once. Remove the cake before the hamster begins to eat it and repeat the process to garner yourself as many stars as needed. This cheat can be repeated as many times as desired to reach the maximum allowed stars in the game. This glitch works on the premise that the day you buy your new hamster is actually the hamster's birthday. The birthday cake gives quadruple the stars on a hamster's birthday. This also works if your hamster baby is born the same day.
Strategy guides from GameFAQs