The Oscar nominee and her mum Betty Lou Motes were not pictured attending the memorial, which took place on Saturday in Santa Monica.
The service was attended by Glee star Jane Lynch and other co-workers of Nancy, who was a production assistant on the musical show.
Julia and Nancy had the same mother, but different fathers; Julia is the daughter of the late Walter Grady Roberts, and Nancy's father is Michael Motes.
The two sisters had a complex relationship; they were very close growing up but in the days leading up to her death, Nancy spoke out against Julia in a series of angry tweets.
The production assistant was said to be battling an addiction to prescription drugs.
According to her fiancé, Nancy also struggled with Julia's fame, and was upset by the actress' departure from the family home for Hollywood when her sister was nine.
"Nancy was proud of Julia but she had severe abandonment issues,' said John Dilbeck, who was engaged to Nancy.
"She had a knee operation as a teen and piled on weight because she couldn’t exercise. While her sister was the Pretty Woman, Nancy was an overweight, insecure teenager."
"Nancy was a tortured soul. She grew up in the shadow of a superstar as a sister and Julia's fame cast a cloud over her life."