Just days away from Ashlee Simpson’s due date, sources tell Life & Style the pregnant star's in a major and very bitter fight with her older sister, Jessica Simpson.

The two have been at it for weeks, according to sources, but what really put her over the edge was Jessica’s two-week 35th birthday trip to St. Barts with a group that included her mom!

ashlee simpson

“Ashlee could give birth any day now,” an insider tells Life & Style of the singer, due in late July. “She’s very upset that Jessica would plan this big trip with their mom right now, during her time of need.”

Another source explains that the fight originated when Ashlee, 30, showed Jessica the new baby’s nursery. “She wanted her opinion, but she didn’t think Jessica would tear all her choices apart,” says the source.

ashlee simpson

Ashlee is expecting her first child with husband Evan Ross.

“Jessica criticized everything from the furniture to the baby clothes. Ashlee had decorated the nursery herself and she was very proud, and Jessica really hurt her feelings.”

Things degenerated from there, with Ashlee lashing out at Jessica for drinking too much and criticizing her parenting skills. Before long, the sisters were screaming at each other.

“They both said some things they’ll regret,” the source tells Life & Style. “Their mom wishes they’d make up, but it’s probably best they spent some time apart.”

The Simpson sisters are close, but they’ve often clashed in the past. In 2004, Ashlee even publicly expressed her insecurity in her song “Shadow.” “It’s about feeling second-best,” Ash explained, adding that her parents and sister “bawl whenever they listen to it.”

For more details on the Simpson sisters’ feud, pick up the latest issue of Life & Style, on newsstands now!