In addition, both Aretha Franklin and Stevie Wonder will sing at the service, and Houston's record producer from the very beginning of her career, Clive Davis, will also speak, Whitney's rep confirms.
According to the family source, "Whitney's family is devastated, but they're getting through it."
In addition, says the family source, "Clive has been a complete rock for her family this entire time. He's been with her since the beginning, and he's been everything to them. They're leaning on him for emotional support. He talks to them hourly, and he's a very big part in planning her funeral. Clive has been a constant presence for them. They're thanking God for Clive."
As the rep told PEOPLE earlier, Houston's invitation-only funeral will take place Saturday at noon at the New Hope Baptist Church in Newark, N.J., where the pop queen's powerful voice was first heard in the choir.
Costner, now 57, and Houston costarred in her movie debut, 1992's The Bodyguard, in which she sang "I Will Always Love You." Houston played a music star threatened by a stalker, and Costner played a former Secret Service agent who protects and falls in love with her. The movie made more than $400 million at the box office.
Houston, 48, was found dead Saturday in the bathtub of her suite at the Beverly Hilton Hotel, and her body was returned to New Jersey on Tuesday.
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The Associated Press will be the only video camera allowed inside Saturday's funeral, and PEOPLE.com will have a live stream from Newark