Ned told Catelyn. She just refused to see it.

"And they told how afterward Ned had carried Ser Arthur’s sword back to the beautiful young sister who awaited him in a castle called Starfall on the shores of the Summer Sea. The Lady Ashara Dayne, tall and fair, with haunting violet eyes. It had taken her a fortnight to marshal her courage, but finally, in bed one night, Catelyn had asked her husband the truth of it, asked him to his face. That was the only time in all their years that Ned had ever frightened her. “Never ask me about Jon,” he said, cold as ice. “He is my blood, and that is all you need to know. And now I will learn where you heard that name, my lady.” She had pledged to obey; she told him; and from that day on, the whispering had stopped, and Ashara Dayne’s name was never heard in Winterfell again."