Ashlee Simpson is ''so proud'' of her sister Jessica for sharing the ''truth with the world'' in her new memoir.

The 35-year-old singer-songwriter praised her older sibling for becoming a best-selling author with her new autobiography 'Open Book' - in which she shared her high-profile relationships with Nick Lachey and John Mayer, her battle with booze and how she was sexually abused as a child - over the weekend.

Taking to her Instagram account, Ashlee wrote: ''I am so very proud of my beautiful, brave, kind, loving sister!!! 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 (sic)''

To which Jessica, 39, replied: ''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 (sic)''

The 'Employee of the Month' star later took to her own Instagram page to thank her followers for their support after the New York Times newspaper gave her the top spot on the best seller list.

She wrote: ''This is me singing ''Hallelujah'' down the hall after finding out my truth not only set me free, but set the lives of so many readers and listeners of Open Book free... Meeting thousands and thousands of you these past 2 weeks has given purpose to my life all over again.

''My smile was real, my hugs were true, and these memories I will hold so close to my heart forever.

''Thank y'all for helping my bravery break unexpected records and giving me the chance to be heard as a #1 BEST SELLING AUTHOR!!! This is a moment for all of us to share, always and forever, your best friend...Jess (sic),'' followed by a green heart emoji.

In the book, Jessica admitted that she became hooked on diet pills after she was allegedly told to lose weight by her record boss Tommy Mottola at the age of just 17.

Jessica said that ''he wanted to sign me'' but he allegedly told her: ''You gotta lose 15 pounds.''

When Jessica questioned his remark, he reportedly replied: ''That's what it takes to be Jessica Simpson.''

Straight after the meeting, Jessica - who was 5ft3 and weighed 118lbs at the time - went on a ''strict diet''.

She explained: ''I immediately went on a strict diet. And I started taking diet pills, which I would do for the next 20 years.''

Her relationship with her body became so bad that she was put on sleeping pills because she started hearing voices.

She said: ''I started hearing voices when I was alone at night, waiting for the sleeping pill to kick in 'Do more sit-ups, fat a**.' By the time we got to the release of my second album single 'Irresistible', I was down to 103 pounds. Everyone went on about how great I looked, but I couldn't enjoy it because I was so freaking hungry.''