How much physical intimacy is normal between siblings of opposite sex?

It's pretty common for opposite-gender siblings to experiment with each other in various stages of nakedness and touching when they are very young ("playing doctor"), because they are curious about the differences in their bodies. But this doesn't usually become actual sex, or if it does, it occurs once or twice but not on a regular basis.

A child who exhibits unusually sophisticated knowledge of sexual behaviors, or who attempts to act seductively or overtly sexual toward another person (including siblings and/or other children) may be displaying signs of having been sexually abused.

As for nonsexual physical contact between siblings, it's normal for young kids to cuddle and hug and hold each other the way they might do to their parents or favorite stuffed animal. But once they approach puberty, they become more aware of social rules surrounding such contact, and it stops or becomes much less intimate (hugs are still okay, for example).