You can try by allowing each cat its own litter box, placing each one in a different location so that the bully doesn't have the opportunity to stand guard. Provide a means of escape for the cat being bullied. Cats like high places, so you can give her a cleared bookcase shelf to go to or 2 or 3 cat trees in different places. Allow each cat its own eating area. Try Feliway, a product that releases pheremones to calm a cat - that may help to stop the bullying. Or you can also try making a loud noise, like shaking a can full of coins, when you see the bully bothering his sister. That may distract him.
