Stephanie Pratt Speaks on The Hills, Lauren Conrad

The formerly engaged Heidi Montag and Spencer Pratt have a lot of work to do before they walk down the aisle, says his younger sister, Stephanie Pratt.

"I love them both so much, but I don't think they're ready to get married," she said on Ryan Seacrest's KIIS-FM radio show Monday. "They're perfect for each other, though, Spencer loves her â€" I've never seen him love someone so much."

But when Seacrest asked if the notoriously on-and-off duo would be together forever, Stephanie Pratt replied, "Stay together? I don't know."

She said the couple's biggest problem is that they get "jealous a lot ... One of them is a little nicer." When Ryan Seacrest asked who is nicer, she replied, "Spencer is the sweeter person." Well, that's saying a lot, isn't it.

Would Montag ever cheat? Stephanie Pratt wouldn't say, but if she did, watch out. "I'd kill her!" Spencer's li'l sis vowed.

Stephanie also opened up about reconciling with Lauren Conrad, which we saw on last week's episode of The Hills. She felt compelled to apologize to LC because the two were in the same computer class at LA's FIDM.

"I was like, 'I'm scared to death of this girl!'" Stephanie Pratt said of LC.

"I knew I had to apologize â€" the class is nine weeks! If I hadn't, it would've been the worst nine weeks of my life!"

She said Spencer Pratt was "genuinely mad" when he found out the two were pals and that she went to Lauren Conrad's 22nd birthday party.

On tonight's episode, he "makes me cry hysterically," Stephanie said.

Still, she insisted, "I don't think I was at fault... I did become friends with Lauren and went to her birthday party. If became friends with Audrina Patridge, I'd go to her birthday party, too!"

Also, Stephanie Pratt strives to bring Lauren and Heidi together. In a future episode, she said she gets them to appear in the same room! OMG!

"My best friend since the seventh grade and I had the biggest falling out, and we just became friends again, so I am optimistic," Stephanie said.

As for Stephanie and her relationship with Lauren Conrad, she said "[we're] good friends. We're gonna be taking another class together."