Sane equals a course of thought or action that is consistent with our human nature.
Insanity equals a course of thought or action that defies our human nature.
What is sane and what is insane can only be determined by people who are acting and thinking in a sane manner. By the way, you don’t have to be clinically insane to think or act like a lunatic. Normally sane people often experience episodes of crazy actions or thoughts.
Insanity often masquerades as sanity, all the more so when sane people allow that to happen, as when American slaves were once counted as 3/5 human, or when men are legally authorized to marry men. Insane actions or thoughts can also appear to be sane when sane people are so completely controlled by the lunatic fringe that we are inclined to observe that the lunatics are running the asylum (society itself, or some institutions such as journalism or academia).
I am not a psychologist or a psychiatrist, so I make no pretense to having adequately described the difference except as a layman. Reasoning (if not always reasonable) people can disagree about what constitutes insane actions or thoughts, but generally speaking one party is right and the other is wrong, with individuals or societies suffering the consequences.