That's an interesting idea. I've often thought before that it's very telling that Jack tells Ennis about his understanding of what the Pentecost is and then there's the line about "fellas like you and me marchin' off to hell", but I never really got beyond that and considered why his mother didn't tell him about the Pentecost. From the little Jack says about her and a couple of things from the scene at the ranch with Jack's parents after his death it's obvious she's a very religious lady, and yet as you say, it seems strange that she didn't tell Jack about the Penecost.
I guess compartmentalising the two - Jack and her religious beliefs - could account for that, and I do think it's very telling that despite her religious beliefs, she 'accepts' Ennis, and not only suggests he visits Jack's room (where as discussed previously, she knows he'll find the shirts), but lets Ennis take the shirts (and indeed seems prepared for Ennis's return with a bag handily ready to hand to Ennis to put the shirts in), but invites Ennis to visit again.
There's an awful lot going on there, and a lot of complicated back-story for a character we see on screen only very briefly, and in some ways it's a pity we don't know more about her and her relationship with Jack. That scene at the Twist ranch is obviously very difficult for Ennis, but I think it's difficult for Mrs Twist also, and in her final question (almost pleading?) to Ennis where she asks if he'll come back and visit again, I feel strongly that she sees Ennis as a link with her Jack, and would like to get to know more about Jack's life, through Ennis. I don't think it's ever said outright in the SS or the film, but I get the impression that after he moved to Texas, Jack didn't visit his parents that much (I know OMT talks about Jack's visits and about him talking about taking Ennis to the ranch and the two of them running the place, but I don't think he says how often he visited) and I think probably his mother feels she could regain some of that 'lost' time with Jack by talking to Ennis about him.