Multihyphenate Mary-Kate Olsen is ready for her next title: Mom. The Elizabeth and James designer, 30, and her husband of 15 months, Olivier Sarzoky, "really want a baby," a source close to the couple reveals in the latest issue of Us Weekly. "They have been ready to have a child for a while."

A little one will fit seamlessly into the couple of five years' lifestyle. Olsen is already a stepmom to Sarkozy's two teenagers, Julien and Margot (with ex-wife Charlotte Bernard), and she "absolutely loves" them, says the insider. The former child star — who is the sister of Elizabeth and James co-founder Ashley and actress Elizabeth — revealed to The Edit in March that she regularly whips up family dinners for the French banker, 47, and his brood at the pair's $6.25 million NYC townhouse.

In fact, "low-key" is the couple's MO, adds the pal. "They are so normal and super happy!"

In her interview with The Edit, Olsen shared that relaxation is the key to her happy lifestyle. "You find the thing that helps you relax, and if you don't have it, you have to look for it," she said. "Or you get burned out, and then you're not productive."

For more on Mary-Kate Olsen wanting to grow her family, check out the latest issue of Us Weekly, on newsstands now.

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