Jessica Simpson threw Ashlee Simpson a ''bohemian pool party'' ahead of her upcoming wedding.

The 34-year-old star, who recently married former NFL player Eric Johnson, invited about 20 guests, including her mother, Tina Simpson, to the bridal shower at her Los Angeles mansion over the weekend to help her younger sister celebrate her impending nuptials to Evan Ross.

An insider told ''Someone was on hand to help all the guests make flower wreaths.The bridesmaids picked a throwback photo to decorate the cake.''

Jessica also shared a picture of the ''throwback photo'' on Instagram with the caption: ''all the beauties celebrating Ashlee!!! I love you Ash (sic)''

Ashlee and Evan, who got engaged in March, are reportedly preparing to tie the knot at his mother Diana Ross' mansion in Greenwich, Connecticut on August 30.

'The Hunger Games: Mockingjay' actor, 25, previously admitted The Supremes singer was taking a very hands-on role in planning his wedding.

He said: ''[My mother is] involved in every part of our wedding [and] engagement party.''

Evan also revealed he was so desperate to be married to Ashlee, 29, - who has a five-year-old son, Bronx, with ex-husband Pete Wentz - that they initially considered eloping.

He said: ''Honestly, we could go and get married right now, wherever. We just want to be married. We're in love.''