Daisy Soros — noted philanthropist and sister-in-law of billionaire George Soros — celebrated turning 90 at the Rainbow Room on Saturday night, and brought down the house with her comedy.

A spy said Soros regaled family and friends with quips, saying, “They say I look 15 years younger than my age — but what’s good about looking 75?” And, “When people asked my husband about the secret of our long, happy marriage — he always replied, ‘Masochism.’ ”

A guest told us, “It was an invitation no one could refuse.” The invites included a hand-delivered book, “The Years of Daisy,” complete with 90 fine-art cutouts depicting scenes from the birthday girl’s life. We hear guests were directed to wear black or white — but arrived to find Soros in a “bright red dress” and diamonds with her poodle, Tango, at her side.

Party whiz David Monn created towering white paper daisies as part of the decor. Loving toasts were made by Soros’ sons and grandkids. “But it was Daisy who stole the show,” a pal said.

We hear the invitation has even been requested by the Museum of Arts and Design to be considered for its permanent collection.