OK, so I get a message that as Prussia Hanover is prepared to join............is there something I have to do to accept this. As the next turn, nothing changed?
in the period to 1866, you can align Hannover (so its 'ready') but if I recall it won't join. There is a decision linked to 1865-7 that allows you to form the North German Federation (you don't actually need a war with Austria to unlock this).
there is a second mechanism to form Germany post 1870 even if you fail vs the French, that will need you to have played the various diplomatic options that create support from the German minors