Your half-siblings are also heirs to your father’s estate who are entitled to a compulsory portion. However, the half-siblings are not entitled to inherit from your mother’s estate.
It is best to ask the KESB/CAPA for the advance care directive and either write a new one (with revocation of the old one) or formulate an unambiguous addendum to the existing advance care directive.
Inheritance taxes are regulated by canton. The decisive factor is the canton in which you are resident. Unfortunately, I do not know this. Siblings in the Canton of Zurich have a tax-free allowance of CHF 15,000. Nieces and nephews have no tax-free allowance. Siblings in the Canton of Zurich have a lower inheritance tax rate than nieces and nephews. I recommend inheritance and tax planning in order to implement this optimally.
For the canton of Zurich, you can calculate the inheritance tax here:
https://www.zh.ch/de/steuern-finanzen/steuern/steuern-natuerliche-personen/erbschafts-und-schenkungssteuer.html?calculatorId=inheritance
Advance withdrawals are equalized between legal heirs unless the opposite has been expressly ordered (equalization dispensation pursuant to Art. 626 para. 2 ZGB). You can also issue these instructions in simple written form at a later date without changing the will. It is important that these instructions are later found by the right person (e.g. the executor of the will).
In the case of advance inheritance payments, the issue of compulsory portions must always be considered in advance. If these violate the compulsory portion, they can be contested depending on the contractual provisions (especially in the case of real estate or company shares), even if a compensation dispensation exists.
More about this topic
Is unequal treatment of siblings permissible?
Inherit properly in a patchwork family