Perhaps you should make the effort to quash that icky feeling. Of course we all remember and will continue to remember, the horror of that day, but surely your brother deserves his happy birthday. Do you want him to send him on a guilt trip every year?
I had a friend whose younger brother was born on Christmas Day. Once, when I was at her house for dinner, we were discussing birthdays and I jokingly said it must be a real bummer for him not having a separate birthday. A black cloud suddenly settled over everyone in the room and I was solemnly advised that the family did NOT celebrate either Christmas or his birthday as an older child had died (after a protracted illness) on that day some years earlier. Because she was my friend I said nothing, but after all these years I wish I had.
So damned unfair on that boy!