Mary-Kate Olsen.

Mary-Kate Olsen. Photo: Reuters

Former child star Mary-Kate Olsen is to marry the half-brother of former French president Nicolas Sarkozy.

Let’s go through that again: the former star of Full House and one half of the once-ubiquitous Olsen twins is to marry Nicolas Sarkozy’s half-brother, French banker Olivier Sarkozy.

Even by the standard of Hollywood relationships it's a peculiar pairing.

This is, after all, the town that gave the world Julia Roberts and Lyle Lovett, Diane Keaton and Al Pacino, Jack Nicholson and Anjelica Huston, Barbra Streisand and Andre Agassi and, admittedly a long time ago, Tom Cruise and Cher.

Olsen, 27, and Mr Sarkozy, 44, have been dating since 2012 and share a $US6 million ($6.7 million) townhouse in New York.

He is 17 years older than Olsen who, at nine months old, was cast, along with her twin sister, Ashley, in the role of Michelle Tanner in the long-running US sitcom Full House.

It is Mr Sarkozy’s second marriage; he has two children with his former wife, Charlotte Bernard.

The "Olsen twins" were trailblazers in the "pre-teen" market, establishing a licensing business that put their likeness on clothing, accessories, perfume and make-up, posters, dolls and board games, in addition to a line of videos, CDs and DVDs.

According to business magazine Forbes, the twins are worth a reported $US100 million.

Both Olsen twins are in relationships with older men; Ashley is dating director Bennett Miller, who is 47.