While it's perfectly within your rights to regret the behavior, losing a sibling is generally regarded as a devastating event- almost the worst thing that could happen to somebody, even. Given that, I would hope that people at least understood that everybody has different ways of expressing their grief, and that it is a difficult time for all. I suppose the complicating factor is that death has a very personal dimension to every mourner, and so one's expression could be another's distress.
I know for example, when my father was killed last year, my mother too was distraught. What worried and upset her, however, was my apparent insensitivity- for I seemed almost completely unaffected by it. I remember making an irrelevent joke to a cousin at the viewing and we both started laughing. Now that's really something I'd regret (I mean, what poor form!)...but at least it is water under the bridge.