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Fun fact: Cate from Life Unexpected and Haley from One Tree Hill are totally sisters!

Sisters from another mister, anyway.

So says LUX boss Liz Tigelaar, who gave us the dish on tonight's much-hyped (for good reason) crossover between LUX and OTH.

See what the lovely Shiri Appleby has to say about the big event in the video above, then head inside for more scoop from Bethany Joy Galeotti and Liz...

CLICK: Get this week's TV Spoiler Chat.

Still curious about what to expect from tonight's LUX/OTH crossover? Basically, a few of your Tree Hill regulars are heading to Portland for a big ol' music festival put on my some of your LUX regulars—and we'll let Liz take it from there:

"[One Tree Hill boss] Mark Schwahn was so great about being accommodating," she gushed. "And it actually worked out perfectly in our story because Cate's struggling with stuff and it turns out that Cate and Haley have such a similar backstory that I can't even believe the CW picked up our show. (Laughs.) They both are dorky valedictorians who got knocked up by athletes in high school and both guys are named Nate or Nathan. In the crossover, Haley really informs and helps Cate's story."

And all the fun musical guests don't hurt things one bit. Says Liz: "When I walked on the set, I couldn't believe I wasn't at a real music festival. We ended up getting Rain Perry, who sings our theme song. And Ben Lee is doing three songs and the biggest thing ever is that Sarah McLachlan performs and it is epic. It felt so big!"

We've seen the ep and can tell you it is not to be missed. Plus, not to spoil anything too significant, but there is a knee-slapping Roswell reference by another former WB star, and it is to die for.

Tune in to the start of the CW smashup tonight on One Tree Hill at 8 p.m., then stick around for the grand finale at 9 p.m. on Life Unexpected.