February 11, 2016 04:00 PM

Nicky Hilton Rothschild debuted her baby bump at the amfAR Gala in New York Wednesday night.

Wearing a white lace dress with a sheer cut out around her growing belly, the mom-to-be, who’s expecting her first child with husband James Rothschild, looked radiant alongside her date for the evening, big sister Paris Hilton.

“I am so excited. I can’t wait to be an aunt — my sister is thrilled,” Paris, 34, told PEOPLE.

Gilbert Carrasquillo/FilmMagic

And while Hilton will “for sure” be the cool aunt, she’s not ready to place any bets on whether she’ll be welcoming a niece or a nephew.

“She’s so tiny right now you can’t even tell [she’s pregnant],” Hilton explains. “Usually you can tell with the size if it’s maybe a boy or girl, but we have no idea — and they want it to be a surprise.”

Hilton, who now spends the majority of her time in Switzerland, will be spending fashion week in New York before flying back to spend Valentine’s Day and her birthday with boyfriend Thomas Gross.

“We’re going on a surprise trip. He’s very romantic and he loves surprising me — this is one of his little surprises,” she shares.

— Kristen Caires

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