This thread is dedicated to Katherine!
Wow, thanks, JakeTwist! How flattering! Guess I'd better post on it, then. (I just discovered it now.) Also, my view seems to be different from many of those here.
When Ennis goes to see the Twists, IMO, he's basically coming out to them. As someone else put it, in what I thought was a very plain but beautiful way (much to my chagrin -- and despite concerted searches -- I can't remember who it was):
"I'm the queer who loved your son."
Ennis doesn't tell the Twists he and Jack were lovers. That would be entirely out of character. But Ennis does tell the Twists "I can't begin to tell you how bad I feel" about Jack. Which is literally true -- he feels really bad, but can't talk about it. But Mr. Twist immediately makes clear that he knows what's going on. "Tell you what, I know where Brokeback Mountain is," is his coded/symbolic way of saying he knows Jack was gay. (To clarify, that's not Mr. Twist deliberately speaking in code to Ennis, it's the filmmakers deliberately speaking in code to us viewers: the mountain symbolizes their homosexual relationship, so Mr. Twist's line symbolically implies he knows about Jack's homosexuality). If that's not clear enough, Mr. Twist makes it even more so when he talks about Jack wanting to ranch up with Ennis, and then this spring with another fella. Mr. Twist may be a jerk, but he's not an idiot. He gets it. And Ennis knows full well he gets it.
As does Mrs. Twist, which is again obvious to Ennis. Her sympathy, her gentle push toward the shirts, her nodding approval when he takes them -- he understands what she's saying, and indicates so by the way he responds to her in return: nods, a thank you, a lingering grateful look on the porch as he leaves.
So Ennis understands that the Twists "know." But at that point he doesn't care. He doesn't run out and get himself beat up, as he did on Thanksgiving. He is willing to face Mr. Twist and be courteous despite the old man's rudeness. He even comes close to tears at the news of the other fella, despite what he knows it would reveal. Though protective of Jack (he somewhat shields the shirts from Mr. Twist), he also walks right through the kitchen with them, symbol of his and Jack's relationship.
Since Jack's death, he has realized that his desire to hide his homosexuality and their relationship has ruined his life. In that light, hiding seems a lot less important. What has become far more important is expressing his grief, and doing what he can to honor Jack's memory.