In my opinion, I think the need to fulfill Jack's last wish about his ashes, was so overwhelmingly important to Ennis, that he had to go see them.
I believe he intended on just saying that they were friends, but saw something in Mrs. Twists eyes that made him believe she knew.
Also, Mr. Twist spoke freely about Jack bringing Ennis to the ranch and then another fella, and Ennis still never admitted anything.
Ennis never actually said they were lovers, he said "we was good friends." Also at this point, the grief was so abundant, that this was the least of his problems.