put those provisions in the trust. Not uncommon for a couple to bifurcate the money into a revocable portion to the remaining spouse and an irrevocable portion to go to the kids/grand kids etc. OR just leaving certain assets to certain kids.

If that did not happen that's on his estate planning attorney for not making that an option or drafting a plan that would follow his wishes.

If there were provisions in the trust for children distributions and the MIL is not following them, you can and should contest the Trust and ask for an accounting.