I do think she had an afair with RFK after JFK ended his affair with her.

Why not?

Check out this video of Donald Spoto.
link

She didn't have an affair with RFK and she only met JFK three times of which two were in public. Maybe she had a one night stand with him or maybe not.

Any person who now believes that Marilyn had an affair with RFK and possibly more than one sexual encounter with JFK needs to read a serious biography about her and not the exploitative and lurid ones written by many who only wanted to "cash-in" on her fame. If possible, avoid reading salacious "fictions" like ones by Oates, Mailer and many others. Avoid Bob Slatzer, Jeanne Carmen and, by all means, any utterance about Marilyn by Tony Curtis. Oates book is a novel, meaning fiction, and Mailer admitted years ago that he invented most of what he wrote about Marilyn because he needed "cash" to pay his alimony. Slatzer and Carmen have been proven to be frauds and Tony Curtis was simply a pathological liar. A revised version of Carl Rollyson's biography will be published in Spring of 2014 because his also contains much misinformation and falsehoods. At least Rollyson is honorable enough to correct the errors he made by giving liars like Slatzer, Carmen and others, as he has said, "the benefit of the doubt."