From what you say, your sister-in-law was thoughtless (or even worse) by failing to notify you, even if your families weren’t particularly close and seldom had contact (as you acknowledge elsewhere).
But let me ask you a question: What good will your anger do? It won’t change the past; your brother is still dead, and you still had no opportunity to say goodbye to him. But more than that, it won’t change the future. For the rest of your lives your anger will cut you off from your brother’s widow, and also from his children. This may not bother you right now—but it should, because now should be a time to grow closer, not further apart.
This is why I encourage you to put your anger aside and let your sister-in-law know that you share in her grief. After all, you don’t know what she was going through when her husband died; she simply may have been overwhelmed and unable to cope. A funeral director once said to me that death sometimes brings out the worst in a family. Don’t let that be true of you.
Even if others in your family don’t agree with you, ask God to help you put the past behind you and reach out to this part of your family. Jesus said, “Blessed are the peacemakers, for they will be called sons of God” (Matthew 5:9). Open your heart to Christ, and let His love fill you and then overflow to others.