Going from an only child to the oldest is a tough transition. But Kim Kardashian explains that she and Kanye West are doing their best to ease North into the role of big sister to her little bro , who's expected to arrive in December."We read books with her about getting a baby brother and a new sibling all the time! We always say goodnight to baby brother," Kardashian wrote on her website . "And right now, anything her cousin Penelope does, she does. So it’s really helpful that P has a little baby brother!!!" Kim shared of North's playtime with Kourtney Kardashian's daughter, who seems to be leading by example in the big-sis department."Right now, I’m just trying to spend as much time as possible with North," Kardashian continued. "I don’t have that many more nights with just me and her before the baby comes and changes her world completely. I’m really trying to spend most of my time at home so we have this special bonding time." We think it's safe to assume that when the baby arrives, North West will be rocking a lot of "big sis" shirts. (Maybe Kanye will add a few to his next collection?)