We don't meet Mrs. McLendon until the last part of the story. We know nothing about her other than that she reads magazines and is in a dangerous relationship with a dangerous man, McLendon. She seems to be in the story to show us that a person who is violent in one area of his/her life is often violent in other areas. Like his violence against Will, McLendon frames his violence against his wife as punishment.
It looks as though McLendon hurts his wife because she was awake when he got home, which is something he had apparently warned her against before. Her refusal to heed his warning can be seen as an act of resistance against his domination. But, the point is, she is not comfortable in her own home. She probably stays in the marriage for many of the same reasons Minnie stays in her situation. Yet, the fact that she reads suggests that she has a window on the outside world that Winnie does not. She also speaks – she defends her actions to McLendon. The result is not good, but her active character (in contrast to Minnie's relatively passive character) gives us a glimmer of hope in this otherwise bleak story. If Mrs. McLendon can actively resist an enormous bully like McLendon to the extent that she does, she might just be strong enough to leave him, or otherwise remove herself from his abuse.