The legendary singer is eager for the 29-year-old blonde beauty to marry her son, Evan Ross, at her mansion in Los Angeles.
A source told Life & Style magazine: ''Diana couldn't be more excited for her son Evan to marry Ashlee. She offered them her LA home as the venue, and she's been renovating it for the occasion.''
The couple are planning to pull out all the stops for their lavish celebration in August, even though it will be Ashlee's second stint down the aisle.
The source added: ''It's going to be so over-the-top.''
The Motown legend is also expected to serenade guests at the reception, where she will be joined on stage by her daughter Rhonda Ross Kendrick.
And Diana has pulled out all the stops to make the wedding an A-list affair, fuelling rumours of a special performance by Stevie Wonder and Smokey Robinson by adding them to the guest list.
'The Hunger Games: Mockingjay' actor Evan previously admitted The Supremes singer was taking a very hands-on role in planning his wedding.