Sudden behavioral changes always warrant a vet visit to rule out a physical cause. Even though it's only towards her, it could still be physical because he may feel safe to lash out at her or she may pick him up/pet him in a spot that gives him pain.
Once that has been ruled out, start looking for other things that may have triggered this about face. A change in hours/routines may be upsetting him. After ruling out everything else, go back to the basics with training. He appears to have decided that he is dominant over her and needs to learn that she is not a litter mate he can bully. Consider going back to obedience classes with her taking the lead in his training. Personally, I would also put away the spray bottle, it may antagonize him even more.
In the mean time, for both of their safety, keep them separate. If that means confining him to another room or penned area when they are both present, so be it. You can't let him hurt her and you can't let her be afraid of him.
Good luck.
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