Every brand system looks finished on the day it ships. That is the least informative day of its life.
The test is a stranger
The real test comes later, when someone who was not in the room designs something new. A new product, a new market, a new sub-brand, made by a person who has never heard the reasoning. If what they produce lands inside the system without anyone correcting it afterwards, the system works. If it has to be corrected, you built a habit that other people were copying.
That distinction matters more in groups with several brands than anywhere else. A single brand can survive on taste, because there is usually one person whose taste it is. Four brands under one roof cannot. Somebody will always be working on the one you are not looking at.
A guideline is a request
The usual answer is a guideline document. It gets written carefully, it gets approved, and then it sits in a folder while the work happens somewhere else. Not because people ignore it, but because at the moment of the decision it is not where they are. A rule that has to be looked up is a rule that will be guessed at instead.
What holds is a rule that lives where the work happens. In the tool people already have open. In the template they start from. In the component they assemble rather than draw. The system stops being something to consult and becomes the shortest path.
It works out of convenience. Discipline would not have lasted this long. If following the system is more work than ignoring it, it will be ignored, and no amount of governance changes that arithmetic.
The part that is hard to accept
A system that only holds while you are running it is a dependency, and dependencies get removed.
Which means the goal is slightly humiliating: build the thing that makes you less necessary. The measure of the work is how little of it anyone has to think about later. Nobody thanks you for a decision that never had to be made, and that is exactly the decision worth building.
I have got this wrong more than once. Rules written before there was enough real work to test them against. The eleventh product is always the one that does not fit, and every exception after that is an amendment rather than something the system anticipated. Now I build the rules from finished work and generalise afterwards.
The version of this with four brands and three legal entities is in One roof, four brands.