Do you already have EINs for the trusts? Grantor trusts generally become irrevocable and their own separate entities upon the deaths of their grantors.
A Form 1041 should be prepared and filed for each trust in order to report the transactions, but no attachments need to be filed with explanations.
You can list all of the beneficiaries in the course of preparing the Form 1041 and each will receive a K-1 for their share of the proceeds.
If you have absolutely no experience in trust accounting or tax preparation (and your siblings have a tendency to be vindictive - no offense), you will be infinitely better off seeking professional tax guidance and, perhaps, return preparation.
