It's not a matter of being strong or being weak. Some people have said it already, coping with a crisis or loss comes differently for each person. Not being able to shed tears is still a reaction in itself and it certainly doesn't mean that you are inhuman. There are two things which you've said that prove that you are indeed human. First, you felt something deep inside even though you weren't able to express it outwardly; and second, your questioning your actions show that you care for your brother. While your thoughts right now may be self centered, I believe that you truly wanted to show your brother that you love him. A truly emotionless person would not give a second thought about any of this.
I've lost three grandparents, a close aunt, a mentor, and a childhood best friend over the course of time since I was in 6th grade; and in each case, I could not cry at their funerals. The only time I truly cried was when I had to put my five year old dog to sleep this past New Year's Eve. Naturally, I questioned my own humanity, too; but while I still do not fully understand why I'm the way I am, I know that I loved these people (and dog) because I often find myself crying in my sleep from a dream I had of them.
For you, though, your brother is still alive; it truly was a miracle that he survived. Even though you couldn't cry for him then, you can still be there for him now. I don't now how close you are with your brother, but you may want to talk to him about all this as well. Tell him that you do love him.