AS MARY-KATE OLSEN gets ready to enter a trial mar­riage with her much older boyfriend, her twin sister ASHLEY is warning her to slow down.

The former “Full House” star, 26, “plans to share” a historic Manhat­tan townhouse that boasts nine fireplaces with her 43-year-old investment banker beau Olivier Sarkozy, according to published reports.

But sources say Ashley isn’t thrilled with her twin rushing into a trial marriage with divorced dad Sarkozy, who reportedly plunked down $6.25 million for the gorgeous pad.

“Ashley is incredibly happy that Mary-Kate has found a love connection with Olivier,” a source told The ENQUIRER.

“But Ashley has more experi­ence dating older guys, and she’s warned Mary-Kate to slow down. She doesn’t want Mary-Kate rush­ing things by moving in with a divorced man who has two chil­dren.”

Mary-Kate and Sarkozy, the brother of former French president Nicholas Sarkozy, have been dating since April, but their 17-year age difference has raised eyebrows.

His bitter ex-wife Charlotte was quoted by The ENQUIRER as say­ing she thought it was “grotesque!” that he was robbing the cradle by dating Mary-Kate.

In the past, Mary-Kate – who with Ashley is worth $300 million as co-owner of entertainment com­pany Dualstar – has dated men who are seven years her senior, including artist Nate Lowman and late actor Heath Ledger.

“But Mary-Kate was only 11 years old when Olivier married Charlotte,” said the source. “Ashley just wants to watch out for her.”

Now some wonder whether Ashley will buy the pad’s twin townhouse, which is also for sale.

The source pointed out: “There would be no better way for Ashley to keep an eye on her sister!”