Luther rejects marriage to the wife of the deceased brother as quite unchristian.
From the Ludwig Collection in Halle in Schütze, Vol. I, p. 191 and in De Wette, Vol. V, p. 436. German in Walch, Vol. XXI, 1485.
Since you seek counsel in the matrimonial matter of a brother who married the wife of his deceased brother: Flee these abominations of the devil and do not burden yourself with other people's sins. It is not a question of whether they are to be divorced, but they are to be considered unbelievers (a Christo alieni) if they do not divorce immediately and go to foreign countries full of each other, one here, the other there, for the sake of the greatness of the outrage. Otherwise, there are enough sins by which we are oppressed that we do not have to bear these monstrosities as well. If your Silesians continue to anger God in this way, it is safer to leave them. Fare well in the Lord and pray for me. On Thursday after Valentine [16 Feb.] 1542. Yours M. L., D.
No. 2887.