I don't have any siblings but I'd imagine that growing up together creates a special kind of bond. The kind that makes that person virtually a part of you. And the kind that you need and rely on even if you don't consciously realize it. It's such a cliche but no matter how much you appreciate someone when they're alive, you don't REALLY understand how important they are until they're gone. It's just human nature and it doesn't mean you didn't love them. So losing someone like that is always going to hit you in the gut. There's no way around that.
For what it's worth though, I sort of know how you feel. I've lost pretty much everyone in my immediate family. That was around 20 years ago. I constantly catch myself thinking that they're still around. But a fraction of a second later I realize that they're not, I miss them and there's nothing I can do about it.
Saying it sucks doesn't even come close to what it's like. Try to take some solace in the fact that in your mind and in your heart, they're always going to be with you.