I understand why you posted, sometimes you just have to tell someone something. This is one of those things. You didn't know who to tell, you just knew you needed to tell someone about your experience and feelings, to let it out and get it off your chest.

I’m sorry about your friend. The reason why you feel depressed is because you and him must have been very close at the time, and talking about them (which I’m sure you did) made you remember him and miss him all over again. Its a natural reaction to miss something after its gone, no matter how long ago it was lost. Try to think of the happy times with him and his sister. Those thoughts might make you feel better. As long as you keep thinking of him in a positive light, he never really left your heart.

Again, you have my condolences.