Yes, it's abuse. Abuse doesn't have to be constant to qualify as abuse. In fact, most abusers will switch between being bad and good to create something called trauma bonding. It's all part of the abuse, or the cycle of violence (worth looking that up maybe?).
It sounds as if your sister has very low self esteem. Bullies are people who have low self esteem and make themselves feel better by making other people feel bad.
She needs professional help, but it's unlikely she'll admit that anything is wrong with her.
If she is still a minor, your parents can make her go.
It's terrible that your parents aren't dealing with this for you. They should. You shouldn't be suffering every day.
They would probably benefit from some help also and you as well

I had a traumatic childhood and thought at the time that it wasn't abuse because my mother wasn't hitting me every day. As an adult I found out that it was and it has affected me in many bad ways. For example I married an abusive man, never thought that would happen. And I failed my studies due to a nervous breakdown, even though I did well in school. It's because I hadn't dealt with my trauma yet.

I only wish that I had gotten help when I was younger, so that it didn't come back to bite me later. So I really wish that for you. A psychologist could really help you make sense of all of this and make sure that you're not too badly affected. Make sure you find someone you like though, because some will fit you better than others and it needs to be someone you can trust. Going to a psychologist doesn't mean your crazy at all.
I've been in treatment a few years now and it has helped me tremendously with the effects that the 'crazy' people have had on my life.

If this isn't possible for you now, know that there's a wealth of info online. Just google characteristics of abuse and go from there.

Lastly, this will end. You are not going to live with your sister forever. It might seem like forever, but a few years is nothing in a whole life. You will get out of this and you, being the healthy one, will have so much more chance of succeeding in life and creating happiness than your sister.
It's up to your sister to change. If she doesn't and you do move out when you are old enough, go no contact until she changes for real.
It might seem harsh, but it's to keep you safe.

In the meantime, don't let her isolate you! It's one of the tactics of an abuser (they all follow a similar recipe). Try to create as many activities/friends/times separate from her. Put a password on your phone etc. Don't give her anything, because she will try to use anything against you.

Good luck and feel free to ask me anything