"Sofia is at her breaking point with all of this," a Keeping Up With the Kardashians source told Radar. "Scott basically told her that she was not welcome on this trip."
The insider continued, "Sofia knows that Scott loves her and that is not the issue. The issue is that she believes, and always has believed, that he is still in love with Kourtney!"
As KUWTK fans know, Kourtney, 40, and Scott, 36, jetted off to Costa Rica this week with their three kids, Mason, 9, Penelope, 6, and Reign, 4. Sister Kim and her children North, 6, Saint, 3, and Chicago, 1, with husband Kanye West are also on the family trip.
Meanwhile, Sofia, 20, has been photographed in New York City, where she is said to be working.
According to the source, Sofia was "mortified" when she watched Kourtney's sisters say they think she is still in love with Scott.
"She knows how the Kardashians truly feel about her now after watching the show," the insider said. "All this time Sofia has hung onto hopes that they were completely over each other and just co-parenting."
The source added, "But now that Kourtney is single, Sofia is starting to think that maybe she was just a rebound all along. For the first time, Sofia is starting to truly believe that Kourtney is coming for her man."