[Edi]

Siege situations are actually more complex than that. There are three types of conditions on provinces with regard to this:

1) Full ownership. Your province, no other factors
2) No ownership. Fully independent or full ownership by another nation. Stealth units may be present.
3) Partial ownership. Under siege.

Assassination can apparently only happen when you have no ownership. Some events may only happen when you have full ownership. Some may happen even when ownership is not full. E.g. PD events go to sieger, indie attack events happen against the besieged and thus take place only when the siegers have left or are victorious and the ownership is no longer contested. This last one looks like a bug, but is consistent with event mechanics and conditions. It is classified as a bug and probably needs some adjustment of event trigger conditions, because it used to work just fine in Dom2.

As far as tax rate etc, KO posted once that the besieging army gets tax rate minus castle admin value of the tax money and the besieged side gets castle admin value in money. The besieging side controls the tax rate, though. This also explains why even when your last castle is sieged and you have no other provinces, your income is not 0, as I have personally seen in a vanilla MP game.