You seem to think the house of Saud is one unified entity, it is not. There are thousands upon thousands of members in the house of Saud, filling all sorts of official roles within the government.
They do not March lock step.
If any of those in official power knew and directly financed the 9/11 attacks, that does not mean that all the others knew.. And if they did to stop it would mean they must turn on their own family, perhaps a more powerful member of the family. Also if they turned against the Wahabbists and started to arrest them, then they would likely turn their violence directly against the hous of Saud.
There is a lot more to the political calculus than, oh the US is a big customer.. A lot more.
So what are you saying, that somewhere in the hive that passes for a government in Saudi Arabia, might have been people who knew the 9/11 attacks were being planned?
Well fine, but then how does one hold the “whole thing” responsible for the attack? Most of it will protest, and correctly, that they did not know and were not told.