Act I in this case is the same as Exposition, so it lasts up to the point when Glen molests Bone in the hospital parking lot. This is the moment when everything changes forever, and there is no coming back from it.
Act II is a little trickier. We're going to say that Act II ends after Ruth's funeral, when the family sees the blood on Bone's legs, and her uncles beat the tar out of him. That's because this incident is preparing us for an even greater catastrophe, one that will compel the plot to wrap up by forcing Anney to make a decision.
This is when what we've been waiting to happen—Anney finding out about Glen's sexual abuse of Bone—happens, and we know that something major is going to have to change. You know how they say that a gun introduced in the first act will be fired in the fifth? Think of the abuse as that gun: when it goes off, we know that it won't be getting any worse. So, naturally, after things have gotten as bad as they can possibly get, we know that resolution is going to come quickly.