Kristi,
How about black or red shirts with the denim. I don't think the denim needs to be the same weight or the exact color...just both in light denim or both in dark denim.
CWD Kids has great brother/sister clothing. It can be pricey. It's an Internet store. Get on the mailing list and you'll get notices of sales. www.cwdkids.com
I found it frustrating when my kids were younger and I wanted matching / coordinated outfits. The Children's Place has coordinating clothes for same gender, as in 3 girls of different age groups, but not for brother/sister.
Good luck and have fun with it. I've taken the kids for plenty of photos where they were both in their favorite outfits, and as long as they didn't clash, they still looked great!