Gunman's Bluff by Edgar Wallace

It came as something in the nature of a shock to Margaret Maddison to discover how completely changed were her feelings toward the man with whom she had passed through stages of toleration to liking, and from liking to a sort of passive affection, and from that again, in the cataclysmic revolution of feeling that her brother's death had brought about, to the bitterest loathing.

For the first time in her life Margaret was in love, and in love with something which was neither a memory nor an idea, but something which was to her as real as her own hand. She had gained that sense of possession which is the wife's own sense -- an understanding of her obligations. She could not afford to waste time in regrets at the amazing follies and wicked errors of the past: in the days that followed her mind was occupied with schemes for helping him out of the morass in which he struggled.