Jessica Simpson stormed out of the party to mark her sister Ashlee's West End debut on Monday night, the Mirror reports.
The paper claims that the actress and singer, who has so far overshadowed her younger sibling's professional success, left the bash at London's Sound Bar fuming: "I don't want to be here - why won't people just leave me alone?"
Her father Joe subsequently told the Mirror that his eldest daughter, who split from husband Nick Lachey last year, was "quite down".
"She's not comfortable talking to people as she's quite depressed and we're walking on eggshells," he lamented.
Reports indicate that Jessica's behaviour following Ashlee's first performance as Roxie Hart in the stage show Chicago wasn't fuelled by jealousy of her sister's new-found fame.
The star, who flew to London especially to watch Ashlee perform, admitted that she had cried with pride at seeing her tread the boards.
"I was weeping. I was so proud. I couldn't be more proud of her," Jessica told BBC Radio 1 following the show.