I think after Anakin turned to Vader, that Old Obi or Yoda hid the twins in two different locations so that Vader wouldn't raise them up to use the dark side of the force.

"If you strike me down now, I will become more powerful than you possibly could imagine!" What did Obi mean by that? By striking him down, Luke would be forced to finish training and come back to battle Vader? Isn't that like hate, and the wrong side of the force? FBog!