Entertainment Music Pregnant Ashlee Simpson Celebrates Sister Jessica's 40th Birthday with Cute Throwback Photo "You are forever my best friend and inspiration and I'm so lucky to call you my sister," Ashlee Simpson shared By Georgia Slater Georgia Slater Twitter Georgia Slater is a writer/reporter on the Parents team at PEOPLE. People Editorial Guidelines Published on July 11, 2020 12:00 PM Share Tweet Pin Email Photo: Ashlee Simpson/Instagram Ashlee Simpson Ross is taking a trip down memory lane in honor of her older sister Jessica Simpson's 40th birthday. On Friday, the pregnant star, who is expecting her third child, posted an adorable throwback photo of the Simpson sisters taking a bubble bath together when they were little girls. In the sweet image, the siblings are covered in suds as Ashlee flashed a smile for the camera while Jessica put on a grumpy face. "Happy Birthday big sis!! You are forever my best friend and inspiration and I'm so lucky to call you my sister 👯♂️," Simpson Ross, 35, captioned the birthday tribute. "I LOVE YOU 🥳." While her milestone birthday was on Friday, Simpson commemorated the occasion a day earlier by trying on some vintage jeans from decades ago. "I have kept these throwback True Religion jeans in my closet for 14 yrs (I’m not exaggerating!)," Simpson wrote on Instagram, sharing a mirror selfie of herself rocking the denim, which she paired with a black and purple sweatshirt. "I figured that since I’m in the final hours of my 30’s I’d give them another try, and hello 40, so nice to meet you," the mother of three added. Jessica Simpson Marks the End of Her Thirties by Getting Back Into Her 14-Year-Old Jeans Many of the birthday girl's followers couldn't believe how fabulous she looked at 40. "You look about 15 lady 👌🏻," replied January Jones. "Wow. You look so beautiful. Happiest. ✨✨," added Jenna Bush Hager. Last month, Simpson said "YEE-HAW" to the "final days" of her 30s by donning a cowhide-print bikini and embellished black hat as she posed for a photo on the beach. Jessica Simpson Posts Cheeky Bikini Photo Ahead of 40th Birthday: 'Final Days in My 30s' Earlier this week, the Dukes of Hazzard star also celebrated her wedding anniversary with her husband Eric Johnson. The couple rang in six years together on Sunday night, and Simpson shared a sweet tribute to the former NFL player on Instagram, saying that she married her "perfect soulmate." "Eric Johnson, my Husband, I love you. 6yrs ago today I married my perfect soulmate," she wrote. "Our unity was written in God’s sky of colliding stars. Together we manifest dreams, nurture desire, and hold space. Destined, it always has been and always will be, forevermore." She concluded the post with a quote from Emily Brontë's Wuthering Heights: "Whatever our souls are made of, his and mine are the same." Simpson and Johnson share three children, Maxwell Drew, 8, Ace Knute, 7, and Birdie Mae, 15 months.