People:

Simpson and her NFL-star beau were spotted together Friday on a sushi run in L.A., and later he was by her side again during Ashlee's rehearsal dinner, held at Jessica's Beverly Hills home.

"They were very cozy and cute together," said a source close to the couple, slamming Internet reports that Simpson was devastated to learn that her ex, John Mayer, has taken up with Jennifer Aniston. "She's definitely not mourning John Mayer! She was in an amazing mood and so happy her sister was getting married."

As for Saturday's wedding and Alice In Wonderland-themed reception, Romo continued to shower the elder Simpson, 27, with attention, said the source.

"They were kissing and holding hands throughout the night," added the insider. "He was very sweet to her. They were very much a couple."