Ashlee Simpson Reacts to Sister, Jessica Simpson's Candid Memoir, 'Open Book'

Gettyimages | Kevin Mazur

Jessica Simpson's candid and vulnerable memoir, Open Book, has been receiving a lot of praise for how it's helped other people get through hard times in their lives. In the book, Simpson reveals some of the biggest struggles in her life including her battle with alcohol and substance abuse, as well as, her tumultuous marriage to Nick Lachey and their subsequent divorce.

While it's caused a little bit of controversy in her own personal life, the reception of the contents of the book has been overwhelmingly good, leading people to offer words of strength and encouragement to Simpson, including from her sister, Ashlee, who went to Instagram to share her thoughts.

Gettyimages | Kevin Winter

The shoutout by Ashlee Simpson comes as her older sister's book has hit the New York Times Best Seller List for nonfiction. Taking to social media, Ashlee shared a congratulatory message for sister, ensuring she knew how proud of her she was.

"I am so very proud of my beautiful, brave, kind, loving sister!!!" she began the post. "It’s beautiful to see you sharing your truth with the world. You are so inspiring. I love you so much. You are a #1 best selling author ?."

Gettyimages | Jim Spellman

Jessica was obviously very moved by the kind words from her sister and responded back by thanking her for unwavering support along the way. "I could not have done any of this without your eternal love and endless support!!! Your bravery in life has taught me how to bandage up my wings and FLY ? I love you ?," Jessica wrote in her response.

Not ironically, in Simpson's book, she spoke about how close the two were over the years, including during some of the darkest days of her life.

Gettyimages | Kevin Mazur

Simpson sat down with Entertainment Tonight's Brooke Anderson to discuss some of the bigger themes of the book, which included the sexual assault she repeatedly received from a friend's parent when she would stay the night.

Simpson recalls that it happened time and time again, with no one knowing about it. Finally, she spoke up and told her parents, which prohibited her from ever returning to her friend's house. It's unknown if any legal action was ever taken by the family against the alleged predator.

Gettyimages | Jamie McCarthy

It wasn't just Jessica who was potentially in danger from her friend's parent, though. Ashlee would accompany her to the sleep-overs and Jessica felt it was her obligation to protect her younger sister.

"I lay between them, fiercely protecting my sister from this monster, I didn’t want her to feel as disgusting as I felt," she wrote in her book. "She continued to try to sleep next to my little sister, and I would just scooch Ashlee over and get between them whenever she did. I never let her near Ashlee, but I also never screamed or told her to stop."

"I never wanted my sister to go through the feelings and the emotions and the pain that I was experiencing," Simpson told ET. "[We] were four years apart, so I wanted to protect her even though I hadn't talked to my parents yet about it. I had my ways of making sure that none of this was happening to her."