It's understandable that to save time, sometimes parents will bathe children of the opposite sex together, but after age 3 or 4, this is not a good idea. Sigmund Freud observed that children become interested in touching, exploring genitals and masturbating at around age 4. At age 4, boys can have an erection and an orgasm. Remember that Freud believed that "penis envy" is a big part of what motivates females!
Since brothers and sisters are not supposed to be interested in each other sexually, however, parents might assume that there's no problem in bathing them together. Children, however, whether they are brother and sister or not, are not fully acculturated, are not inhibited by our rules and are puzzled by sexual differences, and they are driven by their curiosity, pleasure-seeking and instincts.
If a boy becomes aroused while being bathed with a girl, the parent may become alarmed and almost invariably will communicate apprehension that conveys to the child that what he is feeling is wrong. Since this is happening before a child can think abstractly and has such linited knowledge of the world, it can become the seed in the noxious problems of tomorrow, a chance not worth taking.
In Europe, children are often nude at the beach, but this is a public setting without the closeness and intimacy of a bathtub at home, so any sexual feelings are less intense and, therefore, less toxic. After age 3 or 4, do not bathe children of the opposite sex together.
I'm Dr. Blokar.
Here are some parenting resources for you: