Inheritance Question

I have a few questions about inheritance. Relevant facts:
I am US citizen. Japan tax resident.
I inherited an IRA worth 48,000 USD from my unmarried, childless aunt who was American and only ever lived in the US.

I cashed out the IRA without ever taking possession of it and the company that held the IRA deposited about 38,000 USD into my US bank account. The remainder went to pay taxes to the US government on the gains in the IRA, as all money contributed to the IRA is pre-tax. (The above numbers are approximate values.) I received a 1099-R.

I received 20% of the IRA, so that means the total IRA was about 240,000 USD.

I heard the were other assets that were inherited by distant family members. Information about the amounts and who received inheritance is difficult to obtain.
There were 4 siblings who were statutory heirs.

Questions:
1. Will the NTA view this as both inheritance and capital gains? I've read that there is vagueness in the way this can be interpreted.

  1. How can I find the total amount of all assets? I don't have relationships with those who may have inherited. I don't know if she had a will. I don't know if there is someone or some company to ask. Without knowing the total assets how can I tell if the inheritance needs to be reported. If I report only the IRA of which I inherited 20%, it seems to fall below the reporting threshold. However, is this appropriate or reasonable.

by todayeveryday