För Brödrafolkens Väl!
It requires the forming nation to be a great power, have at least 45 prestige and be at peace. On top of that one have to either directly control or have sphered every single core of Scandinavia. Normally it is enough to have either Denmark or Sweden to be a Great power, and sphere the other one as they control all the cores between them.
The description in-game says:
By Reaching a dominant position in Scandinavia, we can unite the Scandinavian peoples under one banner. We must, in order to do this, have the entirety of the traditional Scandinavia either within our realm, or in our Sphere of Influence. For the good of the brother people!