I always thought that Alma Jr's motive was more that she was worried about Ennis being lonely,rather than because she was unhappy with Alma and Monroe, just by the way she accepts his refusal straight away. Alma says that she and the girls worry about him being on his own, but Junior tells Cassie that he's not the marrying kind, so she maybe feels that it's up to her to take care of him, or at least she wants to reassure herself that he's capable,and content enough, to take care of himself. I can quite believe that Monroe and Alma probably were quite strict with her, as others have said above, but if Junior was that unhappy I think she would've tried harder to persuade Ennis, or being a teenager, she would've sulked about not getting her way, but she just says she understands and seems to accept it. Then again, she already knew that Ennis worked long hours, etc, so maybe she didn't really expect him to agree in the first place, or maybe she just inherited Ennis's "if you can't fix it, you've gotta stand it" approach to life, but my feeling whenever I watch that scene, is that she's more concerned for Ennis than herself.