The legendary singer was overcome with emotional as she witnessed Evan Ross, 26, tie the knot with the 30-year-old beauty in a romantic ceremony held at her lavish home in Greenwich, Connecticut, on August 30.
Diana's daughter Tracee, 41, told 'Access Hollywood': ''She choked up and it was so cute. Halfway through the ceremony she was like, 'Oh my gosh.'
''Then she looked over at me and my sister and of course we were both floodgates. Luckily though my mum managed to pull herself together.''
The 70-year-old star, who is ordained, was heavily involved in the ceremony and even officiated the couple's wedding and then went on to perform her ballad 'Endless Love' at the service afterwards.
Tracee explained: ''My mum performed 'Endless Love.' It was so lovely. It honestly couldn't have been more beautiful to watch my brother get married.
''Ashlee is so lovely and so is her sister Jessica [Simpson]. Well, they're my sisters too now!'
However, it was previously believed there was tension between Ashlee and Diana in the lead up to the nuptials, but the singer insists she was only interested in making her son's big day the best possible.
Diana said after the wedding: ''I want only love, happiness, and joy for my son and his bride.''
Meanwhile, Ashlee who started dating 'The Hunger Games' star Evan last year, was previously married to Pete Wentz, with whom she has son Bronx, five.