"Jess is awesome," Simpson told PEOPLE in the way of a pregnancy update on her sister, who's expecting her second child. "I think we both equally look up to each other ... [She's] my big sister and I'm proud of her."
"And she's a great mom – and a great pregnant lady," Ashlee continued.
The younger Simpson, 28, joined her gal pals for a night out Thursday at Los Angeles's Pink Taco for a Svedka vodka-sponsored "Throwback Thursday" party, featuring performances by acts including All-4-One and Debbie Gibson.
There she rocked out to '90s tunes with her cousin, her best friend and her publicist, wearing an Izmaylova dress with a Lizzie Fortunato necklace, her blonde hair pulled up high in a sleek top-knot. She took to the floor to slow dance with a friend and then passionately belted out retro tunes.
Simpson also told PEOPLE her son Bronx has bonded nicely with Aunt Jessica, 32, and her daughter Maxwell, 8 months.
"Bronx loves her, Bronx loves Maxwell, we're all there," she said of their growing extended family as a new baby is on the way and Jessica works to create a new TV show based on her life.
"She's got all our support, naturally, but she's a strong woman," said Ashlee. "[She's a] woman to look up to and a big sister I'm proud of."
Bronx, she added, is also happy about welcoming another baby into their family.
"He's excited for the new one!" Simpson said.