@kimkardashian/ Instagram

Kim Kardashian Babysits While Sis Kourtney Gives Birth To Baby Boy

Auntie Kim takes the kids to 'Disney on Ice' for Mason's birthday.

Kourtney Kardashian welcomed baby #3 over the weekend. On Sunday, the eldest Kardashian sister gave birth to a baby boy -- who now shares a birthday with brother Mason, who turned 5 the same day.

While Kourtney was in the hospital, sister Kim took over babysitting duties, playing awesome auntie to Penelope and Mason.

"Happy Birthday to the lego master Mason!!! You are so big I can't take it! You are my everything!!!!! I love you so much!!!!" Kim wrote on Instagram.

Continuing to be the best aunt ever, Kim took the kids to "Disney on Ice," bringing along daughter North and Zeplin, the son of friend Joyce Bonelli.

Ciara and baby Future also tagged along.

While Kourtney couldn't be there, she wished her son a happy birthday on Instagram, posting this adorable photo and sweet words to go along with it: "Happy birthday to the coolest little boy in the entire world. You teach me so much every single day and changed my universe the second I met you. You have taught me what life is all about and I am truly blessed and honored to be your mommy."