Any tax or fee for an inheritor to inherit the stocks / money in IBKR?

For US market holdings, the estate tax rate is 40% if the account size is above 60k.

One way to go around this is to try adding your spouse's name to the account.

If you pass away holding over 60,000 USD in US-domiciled assets and cash in any of your US-based banks/brokers, your estate may be liable to pay estate tax on US situs assets. This does not apply to non US-domiciled assets such as Irish-domiciled US ETFs. If you do investments into specific stocks rather than index ETFs, then estate tax may be rather unavoidable should an untimely death occur.

Do note that this treatment with regards to cash may be different if you hold an account with IBKR SG, but you will need to check with them.​​​