But Mark Epstein, the former board chairman at Cooper Union, alleged to Crain's that this was "a mistake."

"There's no business relationship between those entities," the younger Epstein brother maintained.

Nonetheless, Mark Epstein admitted that his brother used a number of apartments at Ossa Properties' 301 East 66th St. condominium complex—but said he did not know who paid the rent for these Ossa-owned units, or what his brother Jeffrey, currently under federal indictment for sex trafficking underage girls, did there.

According to a 2009 New York Post article, Jeffrey Epstein lodged several of his victims at the building while purportedly getting them modeling jobs. And last week Crain's found that one of the ownership entities for the accused's East 71st Street mansion used unit 10F at 301 East 66th Street as its address.

"I don't live in that building, I don't monitor who uses those apartments," said Mark Epstein, who put up his Florida home as collateral for his brother's bail bond last week. "I'm not sure who pays it, but Jeff's been renting there for years."

Mark Epstein said that he acquired the building in the early 1990s from Wexner on a tip from his brother. This jibes with the account of Myles Horn, the developer who acquired the midcentury building in the 1980s and converted it to a condominium. An unknown number of the units remain rent-regulated apartments, state records show.

Horn revealed that Wexner was his business partner on a number of New York City real estate transactions, but a market downturn in 1990 forced him to divest from 301 East 66th St. and transfer his stake to Wexner.

"Everybody was either returning these properties to the bank, or otherwise selling them , transferring them, assigning them to somebody who could afford to carry the large negatives for a period of time," Horn recalled. "Les was able to carry these properties. I was not."

But Horn said the 202-unit East Side structure was the only asset he transferred to Wexner, and added he did not know either Epstein brother or any of their partners or employees. Wexner did not respond to a request for comment.