In the movie Shelter (2007), did Jean know her brother was sleeping with Shaun?

I watched the movie last night and I've been trying to figure it out since. Jean says to Zack "You've gotta stop what you're doing with Shaun". These words were kinda ambiguous to me so I can go either way on it. Also, Zack's best friend and ex gf were able to figure it out so I'm guessing his sis did as well. Please let me know what you think. If you have not seen it, I suggest you watch it right now. It's a good story of "forbidden" love triumphing. Could've used a bit more conflict imho but still good.