If Gustaf attacks your sister at night then maybe giving him a room of his own first of all. I would also take Gustaf to the vet to check the behavioural problem is not linked to any medical issue.

I had a similar kitty attack issue for a little while here when we rescued a stray. I mixed one part white vinegar (about a tablespoon) to at least a litre of water and put it in a spray bottle. Cats hate the taste of vinegar no matter how dilute it is. Whenever Gustaff attacks he gets sprayed with the water. IF he attacks again then I would pick him up and place him in a very boring room (thats safe mind you) for "time out". My kitty has learnt very quickly not to attack Bastet because the pantry is a very boring place to stay for a few minutes.