From what my grandparents told me, the heads of State for many of these states and principalities (Prussia, Brandenburg, Nuremburg abd more) were Hohenzollern since (maybe) 1200 AD. Then, in the middle-to-late 19th Century, Bismark nationalized Germany and, essentially, outlawed Royalty. My great, great, great grandfather came to Pittsburgh as a draft dodger from the "Vaterland."
It was common practice to draft royal heirs into the military, then shoot them for desertion...so the family moved to Iron city.
The Hohenzollerns do date back (in recorded history) to around 1100 in the vicinity of Hechingen, south of Stuttgart. At the time, they were known as the "Zollerns," according to one theory due to their practice of Sun worship from their ancient hilltop.
Anyway, they established an originally modest fiefdom, ruling from a succession of castles on that hilltop, which would from time to time get destroyed due to rebellions and wars and then get rebuilt.
A couple of up-and-coming scions of the family travelled in (IIRC) the 14th century to one seat of power in the German-speaking lands, Berlin. (The other being, of course, Habsburg Vienna.)
Through a centuries-long process of social and political climbing, they finally ascended to the throne of Brandenburg and Prussia around 1700. The first really notable Hohenzollern king was probably Frederick "The Great."
William the first retained Otto von Bismarck, a Junker of minor nobility, as his assistant and eventually chancellor, in the mid 19th century. Bismarck was a master of European politics and statecraft. Having built the Prussian military machine to a position of superiority, he used them in a series of wars to achieve his diplomatic ends. This was culminated by the Franco-Prussian war of 1870, which was a key development in Bismarck's plan to reunite (or gain hegemony over, if you prefer) the disparate states of Germany under a single leader, the new Emperor William I, crowned at Versailles in still-occupied France. This was the beginning of the second Reich the first being attributed to Otto I in the 10th century.
Bismarck, by the way, had no political use for the Austria-Hungary of the Habsburgs in his unification scheme. He considered the A-H empire by that time to be too weak and politically unstable to be a part of the new Reich. A generation after his passing, his view would be vindicated.
When William I died, he was succeeded for only a few months by his son, Francis William, who was dying of cancer.
Then the grandson, William II, ascended to the Hohenzollern imperial throne. William II ("Kaiser Wilhelm II") was an odd person, marked by his impetuousness. He dismissed the then-elderly Bismarck in the 1880s and plotted his own course thereafter.
Although blamed for World War I (often even called the "Kaiser War"), it was actually precipitated by a Balkan revolutionary's assassination of the Habsburg Crown Prince and his wife, triggering a military response by the Habsburgs against the various Balkan principalities. This in turn triggered the entry of the military Big Dogs due to defense alliances, some of them secret, and the "Guns of August" and all that.
William II at first tried diplomatically to put a lid on the explosive situation; failing that, he jumped into the conflict with both feet. The consequences we all know about.
The ensuing war brought down the Habsburgs, the Hohenzollerns, the Romanovs, and most of the lesser remaining monarchies of continental Europe. It was the end of an age, and perhaps the beginning of the end of civilization too.
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As I have mentioned before on this forum, the ancestral castle of the Hohenzollerns south of Stuttgart is an interesting place to visit.
Not as flashy or well known as Neuschwanstein, it is more accessible from southwest Germany. It was essentially in ruins in the early middle 19th century when a Hohenzollern prince from Berlin/Potsdam visited the place and got the idea of restoring it. The rebuilding took place over a generation, being essentially complete by about 1870; some parts going back several centuries, such as the Catholic chapel, were restored.
Mind you, as in the case of Bavarian King Ludwig's construction of Neuschwanstein at the same time, the era of building castles as practical fortifications was past.
Over the past century and a half, the castle has been very little used as a residence, due perhaps to its being all the way across Germany from Berlin/Potsdam, the seat of power. It reconstruction was symbolic and it utilization ceremonial. However, the Hohenzollern descendants retain the right to live there to the present day.
Sorry, but your grandparents were confused.
The Hohenzollerns were a relatively unimportant family, especially as compared to the Habsburgs, till the 1500s or so. They were still second rank princes for another century or so.
The Wittelsbach family ruled Bavaria, the Wettins Saxony, etc.
Most of these guys weren’t promoted to Kings till the time of Napoleon or later. Boney did so to tie the German rulers to him and to narrow the title gap between them and the Austrian Emperors.