Hello mailbox
You have not mentioned your location. We always assume it is NSW, unless otherwise stated.
In NSW, a company does not “own the strata”. Your brother’s townhouse is one lot in a strata plan. The owners of each lot form a company called the owners corporation. The owners corporation keeps a set of by-laws.
Most owners corporations delegate the responsibility of day to day running to a strata manager. If you want to find out what by-law your brother has broken, you should ask for a copy of the strata plan’s by-laws. The strata manager should have these. So ask the landlord how to contact the strata manager. In fact, every tenant should receive a copy of the by-laws when they first move in to the property.
There may be a by-law explaining that you cannot put things on display in the front garden if people outside the garden can see them. It is not about how benign the decorations are. When you life in a strata plan, there are rules which promote a consistent look from one townhouse to the next.