Having set boundaries doesn't necessarily mean you have to be dominant over your sister. It means you have things which are yours and things which are hers and respect for each other's things...maybe you have routines that work, like MWF you get to pick the TV channel and TuThSat she does. Look at the things which are causing fights and see if you can each give some and set a routine. Whoever helps with cooking dinner doesn't do dishes--the other one cleans up after dinner. Having a therapist would help you work through some of these things as it doesn't sound like your sister has much inclination to work with you on resolving these things.

Pulling for you to be successful!