Also, they don't develop every facet in sequence. Some will obviously have to happen before others, (so, they can't talk before they understand speech, etc.), but it's not like there's a checklist and you have to tick off A before you can get to B.
So, lingle, what it means is that most children would, in your example, understand about secrets before getting empathy, because understanding the concept of keeping a secret implies that you understand that people have different minds and don't just know and feel everything you do. Empathy means knowing that other people have thoughts and feelings of their own. Now, some children would understand that before understanding why we keep secrets sometimes, and before having the restraint to to it and keep a surprise secret, say. Some would get both sort of together.
Mind you, that is an entirely different thing to expressing love. My son still doesn't have any empathy at all, or much ability to reason beyond "but I want this, so you must give it to me", but he knows he loves his little sister. Her being there makes him happy, so he loves her, but he can't understand that she wants things that are different to him, and that she has motivations entirely of her own. If she does something, it must relate to him somehow, etc..
It's the reason young children so often say that they don't love you any more, or why some children list the things they love and toys are above siblings. They know the word "love", and that it is a good thing, but they don't really understand it with the same depth that we do as adults.