Marilyn Monroe 'Was Enamored of' JFK but 'Wasn't About to Break Up' His Marriage, Frank Sinatra's Friend Says

A longtime friend of Frank Sinatra's has more to share about the relationship between Mariyln Monroe, President John F. Kennedy and his brother Sen. Robert F. Kennedy.

"She was enamored of him," Tony Oppedisano, Sinatra's friend and former road manager, tells PEOPLE of Monroe and the president, adding that they were "obviously" intimate.

Oppedisano's just-released memoir, Sinatra and Me: In The Wee Small Hours, offers its own glimpse into the late singer's life and inner circle - including Monroe, who remained a friend until her overdose death in 1962, and Sinatra's bond with the president.

Accounts of Monroe's affairs with the Kennedy brothers have been detailed in news reports, books and interviews for decades, often shared by sources after the three had died.

"It was obviously a sexual thing, and I would expect that there were feelings on her side," Oppedisano says now of Monroe's relationship to President Kennedy, citing conversations he had over the years with Sinatra.

"Just taking a look at the footage of her singing her 'Happy Birthday,' that speaks for itself," he says, referring to a famous scene from May 1962.

Oppedisano says "it's not like she was in love with" the president but rather that she "looked up to him."

"She respected him; she admired him," Oppedisano says. "She loved what he was doing with the country, and then to have a physical relationship with him, she found him attractive and vice versa." (Oppedisano recalls driving with Sinatra in California when the latter pointed out a bungalow where Monroe and President Kennedy would engage in "slap and tickle.")

"It's not that there was nothing there," Oppedisano says, "but let's face it: He was the president of the United States, and he was a married man and a father."

That meant there was "only so far that even she would go," Oppedisano says.

"She wasn't about to break up [the president's] marriage," Oppedisano says, "so she wouldn't let it go that far, even if she felt that deeply."

Oppedisano adds: "Frank did say she had a sense of ethics and morals as well that she tried her best to live up to."

There was another Kennedy in the picture: the president's younger brother, who was then serving as his attorney general.

As with much else about Monroe's involvement with the Kennedys, accounts conflict about how exactly they were drawn together.

Monroe biographer Jason Spada told PEOPLE in 2012 that President Kennedy had essentially passed off the actress to his brother after he grew tired of the romance - although "it was pretty clear that Marilyn had had sexual relations with both Bobby and Jack," Spada said then.

Oppedisano tells it a bit differently: "When things phased out with Jack, that's when she picked up with Bobby, and I don't think it lasted anywhere near as long or was as deep as it was with JFK. Bobby was a different mentality all together."

A sense of aggrievement, Oppedisano suggests, may have been motivated her.

"According to Frank again, she felt a little bit betrayed and, yet again, taken advantage of when it ended" with the president, he says. "And to an extent, maybe her taking up with Bobby was to get back at him, and it wasn't so much about Bobby as maybe sticking her thumb in JFK's eye - 'You're not going to be with me, so maybe I'll take up with your brother.' "

According to Oppedisano, Bobby wanted to "impress on [Monroe] that he was as important as his brother," and he would "tell her things that he shouldn't have told her, sensitive information."

But in her final days, Bobby felt "she was unraveling," Oppedisano says.

Her state of mind and the circumstances of her August 1962 death - attributed to a barbiturate overdose and ruled a "probable suicide" by the coroner's office - remain much debated, including whether or not Bobby came to see her shortly beforehand.

The brothers themselves did not survive much longer: President Kennedy was assassinated in 1963 and Bobby in 1968.

For his part, Sinatra believed Monroe had been killed, Oppedisano writes in his new book. "And he never got over it."