I have two stepbrothers that I have had since I was maybe 6? Anyways, my sister can actually look at them as brothers, but I feel nothing towards them. They act like brothers towards me and I do not know how to react because I don't think of them as brothers, in fact, I just think of them as strangers. I feel that it is because when they did live with us I was too young to understand the situation unlike my sister. All I want to know is how I should feel towards them even if I do not feel that way, I could at least pretend to feel like that when they visit. It's just so annoying.

I don't think you're obligated to feel anything towards them. If you don't look at them as brothers, you don't have to fake it. You can't force feelings that you don't feel. :P

Well,you don't have to force yourself to treat them as brothers, as long as you respect them and they respect you, that's all that matters. You may, however, try to communicate with them more and maybe you might find something worth to cherish, like you might find yourself a new best friend or something like that. You really can't escape the fact that they're already part of your family even if it's not by blood. If your sister can do it then why can't you?