"I really want bubbies (sic) – I always have," Miller told the June U.K. edition of InStyle magazine during a baby shower for her elder sister Savannah.
"I mean, I'm nearly 27 and you have to think about these things, don't you?" posited Miller. "And Sav proves you can work and be a mum. It still scares me, though."
The expectant Savannah – who already has one son, Moses – held the baby shower in west London, with a rock music soundtrack playing on a vintage 1950s record player. Guests snacked on with multicolored cupcakes, salads and tartlets, all washed down with Earl Grey tea and rosé Champagne.
Although baby showers are still a new concept in Britain, the Millers are more familiar with them than most, having an American father.
During the bash, celebrity hairdresser Nicky Clarke asked Sienna about her Welsh squeeze, scruffy actor Ifans (best known stateside for his role in 1999's Notting Hill).
"[He's] mad as a box of frogs – but I'm not exactly sane," quipped the actress, who is already certain her sister's baby-to-be is a girl.
Although she has the fame, Sienna still claims her sister inherited all the desirable traits. As she told InStyle, "When I was born, Savannah stole the height, the legs, the boobs, the spirituality."