Two reasons Lou Ann decides not to tell her mother that she and Angel are separated.

From the text:

"If Granny Logan had known they were getting a divorce she would have had an apoplectic. At the very least, she and Ivy would insist that Lou Ann come back home."

"I've been married now for practically five years, remember?" She felt like such a sneak, letting on as though her marriage was just fine. It was like presenting her mother and grandmother with a pretty Christmas package to take back with them, with nothing but tissue paper inside. She had never lied to them before, that she could remember, but something in her would not let them be right about Angel."