When they are young children? When they are grown?

I ask this because I know a brother and sister (13 and 9) who I see routinely kissing each other on the lips. I have seen them kiss hello and goodbye, when they are excited, and to comfort one another.

As far as I remember I have never kissed any of my siblings on the lips, even the ones I am especially close to and the thought of doing so is just not appealing. Which is the norm, kissing on the lips or kissing on the cheek?