If we don't see much of Alma, we see even less of Lureen. Basically, she's just a "little voice" (133) on the other end of the phone. The suspicion, however, is that she's in the same boat as Alma about the boys' little tryst, since her voice is "as cold as snow" (133) when Ennis talks to her after Jack's death.
We know she inherited a business from her father, which gives her and Jack some stability, and that "Lureen had the money and called the shots" (104). That gives her a little more leverage over Jack than Alma may have over Ennis, which might explain why her marriage to Jack has more staying power. Otherwise, she's basically Alma 2.0, there to quietly disapprove of their relationship, and silently twist society's screws tighter and tighter against our two boys.