I have a 6 year old daughter (turns 7 in 2 wks), and two boys, 4 yrs and 2yrs. My daughter takes a bath alone when she feels like it, but often ends up in the tub with the boys because that's where the fun is. I agree with the other posts that as long as everyone is comfortable, and no one is overly interested in body parts, it's fine to bathe the opposite sex siblings together. While we don't run around naked, my kids see both my husband and I nude from time to time (like when all three kids feel the need to visit with me when I shower). I usually try to discretely grab something to cover up, but try to act as naturally as possible. I don't want to draw too much attention. Of course, my kids have been told that they aren't supposed to be naked around the opposite sex if they're not family. When my daughter has company over, the boys know the girls are not to be disturbed in the bathroom for any reason. They all abide by the rules, so I'm thinking it's okay =)
I think you'll know when it's no longer appropriate.