"It's very traditional," Spencer's sister Stephanie told Usmagazine.com Wednesday at the PaleyFest09 Hills panel in L.A. "They want to keep everything white. Heidi's dress is white."
Stephanie says she'll be a bridesmaid. Heidi's sister, Holly, will also be in the ceremony, but Stephanie wouldn't reveal her role.
"I'm hoping shes maid of honor because I will screw up everything!" says Stephanie, who will wear either a BCBG or J. Crew dress. (She plans to coordinate with Holly on wearing either "yellow, ivory or green.")
Will there be drama?
Says Stephanie, "I hope no one objects!"
Audrina Patridge, who plans to attend, thinks everyone will be approving when Heidi and Spencer exchange vows (which will be filmed for their MTV show).
"They've been together for so long, and they are two peas in a pod," she tells Us. "I mean, they are Heidi and Spencer! I can't see either of them with anyone else. Heidi loves him."
Lo Bosworth, who hasn't always warmed up to Heidi, tells Us she is among those thrilled for the two.
"I feel like anyone getting married deserves the support of the people coming, and I will be there to give it to them," she says.
Also attending: Spencer's frenemy, Brody Jenner.
"I am not in the wedding. I don't have to make that decision about being best man or not," he told Us. "I will just be attending the wedding and maybe sitting with Lauren [Conrad] and possibly my girlfriend and just watching it and taking it in."
But Lauren says she isn't sure she's going.
"I think it was very nice of her to invite me," she told Us, "but I don't think she expects me to come."