It’s a good thing that you want to be nice to your younger sister. This may be hard at first, but as time goes by, it will become easy and worth it.
First, try to become friends with her, talk to her as often as you can, listen to what she has to say, try to know what’s happening in her life. Spend time with her, this includes going out to somewhere fun with her and helping her with her work.
Also try to be respectful towards her, when you treat her with respect she will be respectful towards you. Encourage and commend her efforts – either work or school.
If you have arguments with her, do not rush to “win”. Give her time to express herself, try to stop the argument by staying calm.