Alma is an easy face to put on the kind of disapproval that Jack and Ennis face (and oh yeah, the kind that gets Jack killed). But we also have to remember that she's Ennis's wife and has two little girls with him while they're married. So that means she's got more at stake than just her social scruples.
After all, it's not like Alma's got the life every girl dreams of. She's got kids to worry about, and she's married to a man who'll never love her the way she wants him to. It's a lonely life, so it's no surprise when she says things like, "Ennis, please, no more damn lonesome ranches for us, […] Let's get a place here in town?" (40-41). It can't be easy to be married to such a passive man, so it makes sense that Alma's a bit passive aggressive herself. When she catches Ennis and Jack in a steamy embrace, she doesn't hit the roof the way we might expect (and, as his wife, she probably has a right to). Instead she uses her "misery voice" (57) to try and get Ennis's attention. Which, um, doesn't work.
Small wonder, then, that they drift apart and she eventually shacks up with the grocer.
Like Ennis, she has a way of holding in her emotions until they explode. So instead of talking about Ennis's duplicity, she holds it in until she cracks, saying, "Don't lie, don't try to fool me, Ennis. I know what it means. Jack Twist? Jack Nasty. You and him – " (89). All this after they're divorced and he's off on his own. So yeah… bit of a slow fuse on Alma.
As Ennis says though, "Alma? It ain't her fault." (70). She marries Ennis with certain natural assumptions about him, only to have those assumptions overturned in a single moment, right outside her front door. She's upset and hurt but—like Ennis—doesn't quite know how to put it into words. And really, the girl doesn't have a ton of options either. She's married to the man, after all.
Her bitterness grows over the years until their entire relationship is poisoned. That may be Proulx's way of suggesting that prejudice stems from a lack of understanding and communication, instead of just jerky jerks being jerky. We never get the sense that Alma's an intolerant person at her core. But she does say some rather cruel things when he marriage and family are destroyed by the relationship between Jack and her husband, Ennis.