Stefanie Keenan/VF22/WireImage Kathy Hilton and Nicky Hilton Rothschild
"I felt terrible that I could not be there," the 63-year-old tells PEOPLE over high tea for Wednesday's launch of LoveShackFancy's new home collection at her Los Angeles residence.
"Nicky is easy. I fly in [to New York] for all the births. I'll be there holding her hand. It'll be in the next five or six weeks."
This past weekend, Kathy's other daughter, Paris Hilton, threw a blue-themed bear-filled baby shower.
This will be the first boy for Hilton Rothschild, who also has daughters Lily-Grace Victoria, 5½, and Theodora "Teddy" Marilyn, 4, with husband James Rothschild. The two will celebrate their seventh wedding anniversary this summer.
After the baby arrives, Kathy tells PEOPLE she plans to host her own celebration so their West Coast family and friends can meet her new grandson.
"I felt bad, but we're going to do a sip and see for her when she gets back," she says.
Stefanie Keenan/Getty Rebecca Hessel and Kathy Hilton
Kathy is also ready to make sure her other grandchildren know they are loved when they become big sisters.
"We have to make sure to give Teddy extra, extra attention," Kathy says. "They're very, very excited. They're so happy."
Kathy had to miss her grandbaby-to-be's sprinkle as she was helping to launch flower-forward fashion label LoveShackFancy's new home line.
RELATED VIDEO: Pregnant Nicky Hilton Rothschild Gives 'Bridgerton Vibes' While Cradling Baby Bump
"I had this party which I was not going to be coming in late for," she shares. "I wanted to make sure that everything was prepped and ready and tabletops were clean and everything was ready to go."
Kathy's role as host of Wednesday's tea was thanks to her pregnant daughter.
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Knowing her mother's love for hosting, Hilton Rothschild introduced her to her friend and founder of LoveShackFancy, Rebecca Hessel Cohen, and insisted the two collaborate.
"Kathy is the queen of tabletops," Hessel Cohen tells PEOPLE.