Olsen, 27, and Sarkozy, 44 -- half-brother of former French president Nicolas Sarkozy -- first stepped out together in 2012. Since then, they've been a frequent presence at basketball games in New York City, where they live together in a $6.25 million townhouse.
This will be the former Full House star's first marriage, and Sarkozy's second. He was previously wed to Charlotte Bernard, with whom he has two kids, Julien, 14, and Margot, 11.
"Mary-Kate has made him a better father. She remembers all the kids' special events," a source previously told Us of the husband and wife-to-be. "She thinks he's a great dad."
Her loved ones have a pretty high opinion of him, too. "Mary-Kate's family thinks Olivier is the best thing to happen to her," an insider tells Us. "They love him."
Incidentally, the fashion designer's twin sister, Ashley, is also dating an older man. Following her split from David Schulte, she has been spending time with Moneyball director Bennett Miller, who is 20 years her senior.