'I Didn't Steal Your Boyfriend' singer Ashlee became engaged to actor Evan in January when he popped the question during a romantic holiday to Hawaii.
And this week, the happy couple celebrated that moment with their nearest and dearest at a glamorous party held in Los Angeles.
Ashlee, 29, shared a romantic picture via Twitter showing herself gazing at Evan while standing outdoors on a patio surrounded by fairy lights.
Jessica posted another snap to Instagram showing herself and Ashlee together. "Celebrating my best friend and sister," she wrote. "I love her so much."
Ashlee began dating Hunger Games newcomer Evan, 25, last year. He is the son of legendary singer Diana Ross and will appear in the next Hunger Games movie, Mockingjay: Part 1 as Messalla.
Evan proposed to Ashlee with a five-carat platinum and gold ring containing 140 white diamonds and ruby accents. He worked with LA jeweller Neil Lane to create the band.
After the big moment, Ashlee took to Twitter to share sunny picture of herself and Evan alongside her big news. "My baby love and I are ENGAGED!!!" she tweeted. "Hallelujah Hawaii!!!!!"
She was previously married to Fall Out Boy star Pete Wentz, the father of her son Bronx Mowgli.
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