[...] The nearer the time comes when I shall have to think of preparations for the possibility of my Nibelungs being performed, the more solicitous I am becoming about the casting of the principal female role—the Brynhilde. I still cannot persuade myself that I shall ever find anyone more suitable for the part than she for whom I originally intended it, that is to say, my niece Johanna. Unfortunately, as you know, thoroughly hostile relations have grown up between myself and her father. Though I can otherwise do justice to his good qualities, he has something about him provocative of ill-will in me which, all the more for our close blood relationship, led to my breaking with him altogether. Nevertheless the question for me is whether I need yet quite abandon the idea of Johanna for my purpose, and accordingly I ask you as a really skilled diplomatist to help me to attain my end. As you are friendly with her and her family you would start with a great advantage. I count especially on Johanna's marriage, as that will in some sort do away with her father's interference between us.
If her future husband is a sensible man and has even tolerable generosity of disposition, he is bound to be glad that his wife should close her theatrical career, by co-operation in this production, in a remarkable and unprecedented fashion. If you can get on terms with her and with him, tell them of my wishes and indicate tentatively the summer of 1859 as the time and Weimar as (very probably) the place. Represent to them the extraordinary nature of the affair. [...] I think I shall have to stick to Niemann of Hanover for the "Siegfried"; I should be very glad if you could see and hear him some time. I have got Tichatschek for Siegmund and—if necessary—for Loge. There is still no prospect of a Wotan.—
The first act of Siegfried has just been finished. I am continually very unwell. Still, it will be quite finished in the summer. [...]
Source: Letters of Richard Wagner, Vol. 1, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 330