Ashlee Simpson hosts an exclusive preview of Missoni Havaianas 2012 Limited Edition Collection on June 7, 2012 in Beverly Hills, California. Credit: Charley Gallay/

She's just one month old and already Maxwell Drew Johnson has a wardrobe any grown woman would envy -- just ask the baby's aunt, Ashlee Simpson!

"She is definitely spoiled," Jessica Simpson's sister, Ashlee, 27, told Us Weekly at Thursday's exclusive Beverly Hills preview of Missoni Havaiana's 2012 Limited Edition Collection of her niece, whom Jessica and Eric Johnson welcomed on May 1. "You should see her closet! I'm like, 'God I wish I had your closet!' Her closet is beautiful."

Jessica, 31, has been settling in well to first-time motherhood, sis Ashlee confirms. "I am so proud of her!" Ashlee told Us of her sister, who stepped out for the first time since welcoming baby Maxwell on Wednesday to visit celeb personal trainer Harley Pasternak. "There were small things here and there [I advised her on] that you ask your friends and other mothers [about] but other than that, she's great."

Simpson and Johnson have opted to put their wedding on hold while they savor every minute with baby Maxwell. Simpson, who inked a $4 million Weight Watchers to shed her pregnancy pounds, is also eager to slim down before saying "I do."

"She won't walk down the aisle until she's back in good enough shape that she'll look great in the wedding dress she wants," a source explains.