(Photo credit: Jamie McCarthy/Getty Images

Earlier this month, Ashlee Simpson and Evan Ross, son of music legend Diana Ross, were photographed leaving the Roosevelt Hotel in Hollywood together - and things seem to be heating up!

On Tuesday night, Simpson, the 28-year-old sister of Jessica Simpson, and Ross, 24, attended a party celebrating the Jessica Simpson Collection's fall line together; the singer is the face of the campaign. (Jessica Simpson, 33, who welcomed a son, Ace, last month, was the creative director.)

"Ashlee was there for work, so she was mingling with the guests," a fellow partygoer tells ABC News. "[But] Evan was supportive and stayed by her side. They're a very cute couple!"

See a Sweet Photo of Ashlee Simpson With Son Bronx

Throughout the bash, they held hands and "were affectionate," but mostly stayed low-key with their PDA. "It was cute," the source added.

Simpson was previously married to rocker Pete Wentz, with whom she has a son, Bronx, 4. After her 2011 divorce, she began dating "Boardwalk Empire" actor Vincent Piazza, but they split last year.