The couple is thought to be tying the knot in August (14), and Simpson's soon-to-be mother-in-law will take on the role of wedding singer, according to E! News.
The insider also reveals The Supremes icon will be joined on stage by her daughter Rhonda Ross Kendrick.
Ross and Simpson will reportedly exchange vows at one of his mother's sprawling mansions in either Los Angeles or Connecticut, and fellow music legends Stevie Wonder and Smokey Robinson are expected to be among the guests.
This will be actor Ross' first wedding and the second for Simpson, who was married to Fall Out Boy's Pete Wentz from 2008 to 2011.
She has a six-year-old son, Bronx Mowgli, with the rocker.