I don't think Junior ( or Cassie) knew Ennis was gay.
Often family members are blind to the thought because they love the person so much. As a gay Teen, I spent alot of time with my Aunt on vacations. She was a lesbian but I never knew it. It was only after her funeral that all these other lesbians showed up that I was shocked! How could I of all people miss that? Denial?
My parents had suspected all these years. But I didn't despite all the clues. I loved her for who she was. Her being single all her life was not unusual to me. To me she was just this non-feminine, athletic, lonely spinster living with 30 or 40 cats. Go figure.
So I think it is possible that Junior didn't know. Perhaps she was clinging to the idea her Dad and Mom would get back together eventually? A lot of kids of divorces wish that. After she married Monroe however, that hope was dashed. Then Cassie was just "good enough" yet still in the way of her time with Ennis.
Cassies "I don't get you Ennis DelMar" certainly confirms that she had no clue. Alot of girls who suspect a boyfriend is gay will dismiss the idea because the guy is having sex with them. They figure :"If he was gay he wouldn't want sex right?" Trust me, I have heard that exclaimed from girls before!