But Mildred will not tell the name, and when Tresham says: "Then what am I to say to Mertoun ?" she answers, "I will marry him." This, and no wonder, seems the last and crowning dishonour to Tresham, and he curses, as if she were a harlot, the sister whom he passionately loves.
This is a horrible situation which Browning had no right to make.

The natural thing would be for Mildred to disclose that her lover and Lord Mertoun, whom she was to marry, were one and the same.

There is no adequate reason, considering the desperate gravity of the situation, for her silence; it ought to be accounted for and it is not, nor could it be.

Her refusal to tell her lover's name, her confession of her dishonour and at the same time her acceptance of Mertoun as a husband at her brother's hands, are circumstances which shock probability and common human nature.
Then it is not only this which irritates a reader; it is also the stupidity of Tresham.

That also is most unnatural.