Cameron Diaz, Nicole Richie Together, Relatively Speaking

0207-cameron-diaz-nicole-richie-shopping-los-angeles-SPLASH-01Cameron Diaz went on a shopping trip Saturday with her sister-in-law ... Nicole Richie.

You don't really think of them that way, but it's true ... Cameron's married to Benji Madden and Nicole's hitched up with brother Joel.

Just regular folk enjoying a sunny, warm day in So Cal.