"He swore he'd marry Edith Sheridan, and nothing on earth could stop him!" And then Mary understood.

Her lips parted and she stared at the babbling creature incredulously, a sudden vivid picture in her mind, a canvas of unconscious Sibyl's painting.

Mary beheld it with pity and horror: she saw Sibyl clinging to Robert Lamhorn, raging, in a whisper, perhaps--for Roscoe might have been in the house, or servants might have heard.
She saw Sibyl entreating, beseeching, threatening despairingly, and Lamhorn--tired of her--first evasive, then brutally letting her have the truth; and at last, infuriated, "swearing" to marry her rival.

If Sibyl had not babbled out the word "swore" it might have been less plain.
The poor woman blundered on, wholly unaware of what she had confessed.
"You see," she said, more quietly, "whatever's going to be done ought to be done right away.

I went over and told Mother Sheridan what I'd heard about Lamhorn--oh, I was open and aboveboard! I told her right before Edith.