The Lizzie McGuire actress - who is already a mum herself to two-year-old son Luca - took to Twitter last night to congratulate Haylie and her fiancé Matt Rosenberg.
Hilary Duff (@HilaryDuff) December 2, 2014
She tweeted: "GUYS I'm going to be an auntie @haylieduff & @matt_sts are going to be the best parents to a sweet baby & Luc will have a partner in crime."
Hilary's well wishes to her sister come after Haylie herself revealed her baby news via Instagram.
Sharing a very cute snap of a pair of baby-sized Jordan trainers, the 29-year-old wrote: "Happiest girl in the world! Finally can let everyone know that Matty @mr_sts and I are expecting! #BabysFirstJordans."
Following her Instagram post, Haylie also shared her pregnancy on her own personal blogging site Real Girl's Kitchen.
Posting a blog entitled 'And baby makes three...' the Material Girls star told her fans she and Matt are expecting their first child together, and even revealed the sex of their unborn baby.
"Today we have big, exciting, wonderful, magical, amazing news," she said. "Matty and I are expecting a baby girl."
Haylie added: "Entering this new chapter of our lives and our relationship has been so special. We are so happy and so in love with her already!!!"
The couple's baby news follows Haylie and Matt's engagement earlier this year.
Back in April, the actress and food blogger announced she was to become the "future Mrs Rosenberg" on Twitter.
Revealing she was set to tie the knot, Haylie shared a photo of herself cuddling up to her entrepreneur beau while proudly wearing her engagement ring.
Telling fans about Matt's proposal she said: "The moment was genuine and sweet (like him!) and I couldn't wait to say yes!
"We are so excited and happy to share our wonderful news with everyone! We both feel so blessed to have the love and support of our family and friends and can't wait to start this new adventure!"
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