So, it comes as no surprise that as soon as the news broke less than a week ago that 29-year-old Keira is expecting her first child with husband James Righton, Sienna began being questioned about her best friend's happy baby news.
Sienna — who has a two-year-old daughter named Marlowe with her long-time partner, Tom Sturridge — has insisted, however, that she won't be piling on the advice to Keira as she reckons she'll be just fine.
Well, more than fine actually...
In fact, Sienna's so sure of her friend's mothering abilities that she thinks she'll be the 'perfect' mum. How sweet is that?
The Brit actress, who has recently been Stateside promoting her new film American Sniper, revealed that she and Keira have '...spoken a lot and I know she will be amazing.'
She also went on to say: 'She doesn't need any advice. She will be perfect.'
Keira has made no secret of her desire to have a family in the past but even for the most natural of mothers, the realisation of actually being pregnant and having a child can overwhelm the best of us. But, to echo Sienna, we reckon this new 'role' for Keira is the perfect fit. She just is a mother, isn't she?
Now, ladies; if you need an extra hand planning baby showers or the such like, we're totally available. Just a casual FYI.