What do we do.

We are really at a loss here as to how to deal with the 3 year old in the house. No matter what you do to discipline him when he does something wrong nothing seems to work. Yesterday he was caught in his baby sister's crib jumping up and down while she was laying there. This morning he was caught doing exactly the same thing. We are worried in the first place that he will accidentally jump on her and seriously hurt her or even worse kill her. But no matter how you discipline him he will go back and do the same thing over and over again. Spanking his does nothing at all. Neither does standing in a corner. So right now he is sitting on a chair. But, that doesn't seem to actually bother him either. I asked him why he was on the chair and he told me because he was jumping in his sister's crib. But then he said "you didn't see me". I have never in my live dealt with a child like this. He has no regard for right and wrong.