Well, let's look at your options. You can continue on as you are doing, being charged at by your rooster, having to watch your back and having a bullied, stressed out flock while you're trying to "rehabilitate" him. Or you can cull (which simply means to "remove from the flock" not necessarily...
Unfortunately, sometimes we have to make hard decisions in chicken management. How old is your sister? Old enough to defend herself from him, or young enough to get seriously injured? (As in, potentially getting spurred in the face leaving her scarred for the rest of her life, or possibly...