Sienna Miller's 11-year-old daughter Marlowe "initially" found it hard adjusting to life with her new baby sister

Sienna told E! News: "Initially, it was a lot for her, and now she is in heaven.

On the bond the girls are building, the 'Alfie' star said: "Now that the baby can react, [she] is clearly in love with her big sister."

Sienna has been busy promoting her latest film, 'Horizon: An American Saga', all over the world, and that teamed with having a baby means she is in need of lots of sleep.

Asked what her plans for the summer are, she replied: "Maybe a bit of sleep. I have a new baby."

Meanwhile, Sienna revealed Marlowe was keen to make a cameo in the film, which also stars Kevin Costner and Sam Worthington, but she "chickened out".

Sienna said: "But what's so lovely about this set is anyone is welcome."

The 'American Sniper' actress opening up about life with a newborn comes after Sienna previously admitted she felt self-conscious about becoming a mum again in her 40s.

She told Vogue magazine: "I'd love to get to a point where I didn't feel the need to make a joke of my being older and having a baby ... to show I'm in on the joke."

The screen star bared her baby bump in a photoshoot for the magazine, and she said of the decision: "I was nervous about the idea of it ... [But] I was like, I'll have that photo for the rest of my baby's life.

"It's kind of fascinating to fight your own prejudice against yourself. I'm constantly doing that."