That is such a sad situation. I can tell from the language you used that you and your wife must be quite upset with your brother-in-law. And he is clearly very upset and expressing it in a way that's making a difficult time even more difficult. He also had to take on a terrible responsibility when he was the only person available.

It might be that there's nothing you can do right now to get him to see things through your eyes. Maybe all you can do at the moment is work on self-kindness, forgiveness and patience on your own and prepare for how to mend the relationship in the future. I have seen these rifts in my own family never heal, so I hope that they will in your family.