The Open Set Condition is fulfilled by an Iterated Function System in the plane $\{\mathbb{R}^2, \phi_1, \phi_2, \dots, \phi_m \}$ if there exists a nonempty open set $V$ such that $\bigcup \phi_{i}(V) \subseteq V$ and $\phi_{1}(V), \phi_{2}(V), \dots, \phi_{m}(V)$ are all disjoint.
I think it would be very natural then to choose, in the case of the Sierpinski Triangle, the equilateral open triangle "with same vertices" as the first iteration of Sierpinski triangle. However, in pdf by Bandt and Wikipedia it is chosen this hexagon... I wonder why would they be complicating things?