There are two things to watch out for:
US estate tax is also independent of the brokerâs domicile. Youâre potentially affected if you hold US-domiciled stocks or ETFs. However, it shouldnât matter whether you hold them at IBKR or Swissquote.
location of the broker is not relevant in this case, US situ assets (individual US stocks, US based ETFs VT, etc) fall under estate tax regulations regardless of the broker domicile (IB, Swissquote, ZKB)
As you say, US situs assets are within estate tax anyway. I wondered whether, say, an Irish ETF could be brought within the scope of US estate tax due to being handled via a US broker.
I would hope not, but I know the US likes to over-reach in certain circumstances 
Example
-Fundsmith UK is an open ended fund. It is UK situ (registered office in UK)
-I own it in my account at IBKR UK , which means custody is presumably in the US at IB US
-The fund owns many US companies
I believe it would be out of scope of US Estate tax. Do you agree (?)
Article in the Daily Telegraph (paywall)
"Currently individuals who are UK domiciled â the UK is their home country â but resident abroad, are charged IHT on their estates globally.
However, under plans to move to a âresidence-based regimeâ from April 2025, expatriates who have lived outside of the UK for 10 years or more will only need to pay IHT on assets in the UK."
Hi Barto,
Thanks for that link. It seems UK Domicile is rather sticky 
So to summarise:
If my mother is deemed UK domicile of origin, it does not matter where the assets are located. Tax will be levied.
If not IB UK will be ok because the assets are located in the US.
Anyway I might contact a legal expert in these matters to see what our options are here.
UK IHT is somtimes jokingly called âvoluntaryâ as with sufficient planning it is possible to avoid it. You can see this old article on how several billions of wealth have been successfully passed down along generations: Inheritance tax: why the new Duke of Westminster will not pay billions | Inheritance tax | The Guardian
Yes, that is my understanding too , assuming that she owns US shares and ETFs
If she is in good health I would not bother contacting a legal expert until the new law is communicated
Not legal or financial advice. Assuming you are resident in Switzerland and not a US citizen, the exemption to the US Estate Tax is decided on your total worldwide wealth, then applied to your US assets only.
Example 1: you own $1M of US assets out of a total of $10M.
You are exempted, your total wealth is below the exception for Swiss residents ($13.61M) in 2024. You probably still have to file some forms to the IRS.
Example 2: you own $1M of US assets out of a total of $15M.
You are above the $13.61M threshold, you will have to pay the US estate tax on your $1M of US assets.
All stocks, ETF, etc domiciled in the US count as US assets. Basically, if you declare them in your DA-1 then they are likely US assets. And some more too (some stocks that pay no dividends are not automatically moved to the DA-1).
Unclear (to me) is how the total wealth is computed with respect to Pillar 2, Pillar3, and real estate property. The IRS probably has different rules than the Swiss taxes.
Be aware that the current exemption will end in 2026 and go back to a previous, lower, value. Unless the next US administration will extend it or change it.
Blog speculating UK IHT may be linked to nationality :
âLong term UK expats are rarely Labour voters so a Labour government is unlikely to want to gift them any favours. We think it is much more likely that it will extend the scope of UK IHT to include anybody who holds a UK passport.â
Caution: the blog is by a firm with a vested interest in people buying trust solutions.