As it is commonly known, cross-linking is a bad idea. But how bad is it? If you look at Crunchgear and Geeksugar, you would notice their heavy cross-linking to their sister sites at their footer area. All without nofollow. Is it something advisable to follow?
I reckon it's okay, because they're in the same category. If I were a context-blind googlebot, I'd think they're in the same category. Plus, they're also held by the same company. The logic against crosslinking is that someone might write a Tourism article in one site, and write about um... math in another site (not me though). Which, is totally unrelated.