Because family trees are designed to show genetic relationships, you will not be able to have your step-siblings next to you AND have the family tree be completely accurate. You can add your step-siblings as actual biological brothers and sisters and have them next to you. You could add them as children of either your mother or father (whichever is the biological parent --if either of them are) and have them near you. Or you can make your Family Tree genetically accurate and have them on the same generation as you, but not so near to you as genetic brothers and sisters.
There's nothing wrong with any of these approaches. You're just trying to use a tool in a way that is different from how it was designed to be used. There are tools like, for instance, Family Tree Maker that will let you move people around and add and remove lines pretty much arbitrarily once you are ready to print a family tree.
