The TOWIE star, 24, is heavily pregnant with her first child and due any day now - but didn't let that stop her attending the fundraiser at Minnies Boutique.
Billie looked cute as a button in a coral maternity dress with gold detail at the bust, with a soft belt tied above her baby bump. And she braved heels!
At one point during the day, Sam - who was rocking a very cool LYDC bag - placed a protective hand on sister Billie's swollen belly and looked like the happiest auntie-to-be in the whole of Essex.
Billie and Sam's Brentwood clothing store, Minnies Boutique, threw a Marie Curie Tea Party on Saturday (28 June).
The soiree, also attended by close friend and TOWIE co-star Gemma Collins, raised money for the charity by selling tea, cakes and raffle tickets for prizes including a Minnies makeover, Fusey merchandise, Ray-Bans and McBusted tickets.
After an afternoon of fun, Billie headed home for a well-earned rest...
The mum-to-be shared a selfie with her feet up at home and the message: "Me and bump having a little rest after a busy day minniesboutique tea party with #mariecurie."
She also posted a heartwarming snap of herself giving fiancé Greg Shepherd a kiss alongside the caption: "Kisses with my love."
The couple confirmed they were expecting their first child in January, and later revealed it's due to be a girl.
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