Your participation in a mutual fund is similar to owning a stock for estate tax purposes. Only US-located property is subject to the US estate tax for non-resident, and the determination whether the stock is US-located property, according the instructions to the form 706-NA, is:
Generally, no matter where stock certificates are physically located, stock of corporations organized in or under U.S. law is property located in the United States, and all other corporate stock is property located outside the United States.
If the fund is a RIC, then this applies:
Stock in a regulated investment company (RIC).
For an NRNC decedent who died after 2004 and before 2012, a portion of stock in a RIC is treated as property located outside the United States in the proportion of the RIC's qualifying assets in relation to the total assets owned by the RIC at the end of the quarter immediately preceding the decedent's death.
Qualifying assets are assets that, if owned directly by the decedent, would have been:
- Bank deposits and amounts described in section 871(i)(3),
- Portfolio debt obligations,
- Certain original issue discount obligations,
- Debt obligations of a U.S. corporation that are treated as giving rise to foreign source income, and
- Other property not within the United States.
See section 2105(d) for details.
So it doesn't matter where the brokerage is located, what matters is where the fund is located (organized). You'll need to check the fund documentation, and to determine how and where it is organized.