"If dear sweet Mrs. Dashwood were ever to hear of his being so near to her as Plymouth and NOT coming by, she would be hurt."

Edward had absolutely no reason to think that the Miss Steeles would ever meet Mrs. Dashwood. But he had no idea that they were relations of Sir John's mother in law. Or that Sir John was so gregarious that he would invite a such a distant relation to visit at the drop of a hat. [Although he probably wouldn't have been surprised that they would drop everything to race off to the home of a baronet when invited.] So, it's a good thing that he made the visit as Mrs. Dashwood might have actually found out that he was in the neighborhood when the Miss Steele's visited.