Depends on your culture, I think. In some families, there is no problem with a kiss on the lips between family members, and they can and do distinguish between what would be a chaste and an erotic kiss (I shan’t go into details). Others, such as mine, I’ll kiss my Mom, everyone else gets one of my trademarked Johnny Hugs™, which, on the proper occasion, is extended to close friends. Some families, I have noticed, make do with handshakes*, especially between male members of the family.
I don’t have sibs, so I can’t give any personal experience there.
I don’t think there is a hard and fast rule (e.g. kiss cannot last longer than 0.75 seconds, you must have your eyes open, etc :rolleyes:). If we assume kissing your sib might be OK, then we can assume that air-kissing next to the cheek is always and the From Here To Eternity beach scene is always :eek:.
As for all the other stuff – I’d say once puberty kicks in, or if any of the kids becomes modest, or if any get to the point of being like Radar O’Reilly, who once said “Nudidity makes me breathe funny.”
*Or punches on the arm.