The couple confirmed plans to marry last week in what Pitt's publicist Cynthia Pett-Dante described as "a promise for the future".
Aniston, who recently admitted to being unnerved by the media's obsession with her 2005 divorce from Pitt, is said to not have been fazed by the news of his union with Jolie as she is focused on her own relationship with boyfriend Justin Theroux.
"She doesn't care. She really doesn't," a source told E! Online.
"She's happy with Justin. She'll probably marry him. She's moved on. People don't want to believe it, but she has."
Pitt previously said that he wasn't living an "interesting" life while married to Aniston, though he later clarified the comments, insisting that he still considers her to be a good friend.
> Brad Pitt: 'I won't wed Angelina Jolie until there's marriage equality'