Spencer Pratt didn't tell his sister he is going to be a father.
Asked if she is excited about the baby, Stephanie said: ''Of course, although I had no idea Heidi was pregnant.
''Spencer and Heidi tend to do things in their own way. I'm really happy for them both.''
However, Stephanie believes the pair will make great parents.
She added: ''Heidi has wanted a kid for 10 years, she's born to be a mum. Spencer and I have nieces and nephews and he's very good with them.
''He always acts like he isn't interested but he's always got his phone out taking videos of them.''
The 31-year-old star - who is dating rugby player Ollie Lindsay-Hague - isn't ready to settle down yet as she enjoys shooting reality TV show 'Made in Chelsea' and wants to be on the programme forever.
She said: ''[I want to settle down] eventually but right now work is my main priority. All my friends back home have kids and live in lovely houses, and when I see them I'm like, 'Should this be me?'
''It's just not something I want right now - I want to travel the world first. I never want to leave [the show]. I want to be on there until I'm 40 and people are like, OK, this is getting weird now.''
Stephanie previously starred alongside her brother on 'The Hills', so she found it easy making the transition to 'Made in Chelsea'.
She told OK! magazine: ''Being on camera is second nature to me, I've done it for so many years.
''I really love the British sense of humour so I'm in stitches every day. In LA people take themselves so much more seriously so 'Made in Chelsea' is way more fun to work on.''
From 'Happy' to 'Banana Pancakes', these are soaked in positivity.