I don't think Lureen would have been as hostile toward Jack as Alma was Ennis; I definitely think she would have kept it under wraps.
Do you really think, Alma was "hostile" towards Ennis?.....she kept her secret of knowing quite a long time, obviously bottled it all up, and it was probably the straw that broke the camel's back as far as any chance that her marriage would survive, but it seems like a few years had passed before she finaly went "hostile" after the thanksgiving dinner....I think then, she felt "safe" being able to say something, she was now remarried, pregnant and was not as afraid of the repercussions of bringing it out into the open, as she would have been if she was still living with Ennis......I think she was remarkable, that she held it in for as long as she did...
If you notice what was said, between her and Ennis before she started on the "Jack Nasty" thing....she said to Ennis he should get married again, and his reply was "Once bitten"...which was a smart arse way of saying that SHE was the one who had turned him off women.....letting him know that she knew about him and Jack, was her way of saying, "its not my fault, have a look at what you have been doing"......and "I know too, what you have been doing"......she was letting him know, that she had no guilts about their marriage failing, and she wasnt going to let him blame her....she didnt threaten him with exposure or anything, she just wanted him to know that she knew, finally........