When she was younger, she knew something. Kids sometimes know! When she was just a little kid, she saw her father go off on these trips, and right before he went on one of his trips was the only time she ever saw him happy. Then when she 11 she saw Jack and how he hugged and touched her father. She began putting the pieces of the puzzle together. Also, after her mother married Monroe, she probably heard them talking about her father sometimes. Alma told Monroe for sure. Therefore Junior and her sister, like all kids, would overhear conversations sometimes. By the time she was a teenager, she knew 100% that her father was in a relationship with a man. She tells Cassie off because she knows her father has been living a lie all these years. She sees how unhappy he is with a woman, and she basically tells Cassie "don't waste your time", when she says "he's not the marrying kind".
She knew why her father wasn't "the marrying kind"...it's because he could never be in love with a woman, and hence any marriage he would have would be a lie and wouldn't work out. Junior knew Jack was the REAL person her father was married to. Of course, it couldn't be an official marriage, but she knew the relationship he had with Jack was that of lovers, not just fishing buddies, friends. She knew they weren't just two guys who go fishing together, there was something far more to their relationship. At the end, when she tells Ennis that she is getting married, she has a look on her face when he asks her "does he love you?" She looked like she was thinking "Yes, he loves me, like Jack loves you, daddy." I believe Alma Jr. got closer to her father over the years. She was a lot like him, they were both shy, introverted people, and she could relate to him much more than she could to her mother.
She understood that he had a private life that was off limits, like children do. But then when she grew into an adult, and Ennis became more accepting of himself, I am sure they talked to each other about Jack. All his life, Ennis shared how he felt about Jack with no one, because he thought they would be ashamed of him if he did, but that all changed when he met Jack's mother. She was the first person who knew approved of his relationship with Jack, and that made him realize many things he should have years ago. He stopped being so ashamed of who he was and his relationship with Jack. By the time Junior was grown-up, he could be much more open with her about the truth. She understood, and became the second person in his life to approve. Ennis learned that there are people out there who are not homophobic and who don't look at his relationship with Jack as wrong, Jack's mother and his daughter Alma Jr were like teachers for him.
