Brad Pitt famously fell for Angelina Jolie on the set of 2005's Mr. & Mrs. Smith and split from then wife Aniston.
Angelina dramatically filed for divorce from Brad in 2016, and Jen and Justin announced their split in early 2017.
"They've been secretly dating for a few months and their romance is heating up," a source says of Angie and Justin.
"To Angelina's surprise, she's actually fallen for Justin."
Another source adds, "Jen still doesn't know about Justin and Angelina, but if she were to find out she'd go absolutely ballistic."
"Not because she still has feelings for Justin, but more due to the fact that Angelina has already stolen her man once, so for history to repeat itself with Justin would be too much for her to handle."
"It'd bring back memories of the hell Angelina put her through during her divorce with Brad," adds the source. "It could easily send her over the edge."
Angelina and Justin are yet to comment on Life & Style's report.