My best guess is that the family was deeply embarrassed. During these times people make decisions that grow out of shame and insecurity rather than out of good judgment.
I believe the correction is reaching out to your brother-in-law. As long as your brother-in-law knows that your husband has told you, and that it wasn’t meant to be kept from you by him, then you reaching out directly to lend your support short-circuits the dynamic of feeling left out.
I would also encourage you and your husband to talk about this. The main dilemma here is that your husband honored his family over you. I believe it is you when your husband that need to talk more about you not being included. As his wife, the separation from this process should’ve been something the two of you discussed prior to his going. But now it will be essential as an effort to heal your exclusion from the process — and work toward integration in the future.