Step-

Can anyone tell me the origin of the prefix "step" in relation terms, e.g. "stepmother"?

It derives from Old English "steop", which is related to Old English "astieped", meaning "bereaved". In other words, you only have a "stepmother" because you have been bereaved of your real one. (This usage pre-dates divorce on demand by a thousand years or so.)

Reminds me of a joke:

First man: Do you have a step-ladder?

Second man: I can do better than that, I have a real ladder.

Rather silly, but it amused me.

DFG