My inheritance was stolen, what should I do?

I am from a well off, powerful family. My father Hugh was a self made man: though he was born into a well connected and well off family, he became really distinguished on his own, and increased the family estate. He was a generous man, too. So, he gave a part of the family estate to his less gifted brother, Henry. The estate was given to him with the understanding that if he did not have any direct heirs, it will revert to our family estate. My father died, and I inherited his estate. My uncle Henry married twice but didn't have any children. He had a stepson from his first wife, called Otto William (a very stupid name, if you ask me, but his grandmother was German, from Swabia). When my uncle died, I claimed the estate as the next of kin and as a rightful owner as per my father's agreement. But Otto William says that my uncle adopted him, so he is now the rightful direct heir! I am sure that it's not what my father had in mind! More so, Otto William seized the estate and manages it as he will! I have my father's estate to look after, and live far from my uncle's estate. So, Otto William used it as an advantage, and now he rules it all! What should I do?