What a sad, sad situation. Your sister has made it impossible for the two of you to have the kind of close friendship that many sisters enjoy. I’m sorry for both of you.
Has it ever occurred to you that your sister is threatened by you? She doesn’t seem to understand that there is room for two girls in the family. She puts you down because she doesn’t feel “up” enough. Her self-esteem is so low that the only way she can see to fix it is to make your self-esteem worse. I feel sorry for her.
I don’t know how this started. With only two years in age between you, she may not have been helped to welcome a new baby sister when you were born. It may be that your parents were so overwhelmed at the time with having two kids under two that they didn’t give her the attention she needed. If that was the case, from her point of view you were an interloper who took away the attention and care she was used to having. (That is not to fault your parents. Most kids in this situation grow out of the resentment and become helpers and friends. We don’t know why she didn’t.)
Now that she is 19, your parents have limited influence on her behavior. They probably feel as helpless as you do. They can try to set some rules for living in their house, but I imagine that she will only break them. Probably, the most useful thing they can do now is to encourage her to get some treatment for her chronic anger.
In some way, your parents are right. There is no way you can change her. There is nothing you can say or do that will help raise her self-esteem from the emotional basement it seems to be in. You are a “bigger person” when you ignore her provocations. Unless and until she gets herself into some therapy, all you can do is drop your end of the argument and get ready to launch into your own adult life.
The coronavirus makes it difficult, I know. Ordinarily, I would suggest that a person in your situation find ways to stay out of the house as much as possible by doing things like getting a job, becoming involved in a community activity, or doing research in a library. But those avenues are closed for now.
The best you can do is “leave” without leaving. That means zooming with friends, getting involved in some household fixing up or reorganizing, etc. or doing volunteer work. For example: People are making meals for people who need it, staffing phone banks, and sewing masks. Get yourself busy with doing something meaningful and the meaningless taunts of your sister will have less impact . Being active will also reinforce your positive self-esteem.
Do start researching what you will do next. You are out of school. Do you want to go to college? Can you start taking some courses now that will give you a jump start? Many schools are now offering free intro courses online. If college isn’t in your thinking, research what you can do now to position yourself for a job you want.
As for when your sister attacks: Just look at her sympathetically and tell her you are sorry she feels that and you’ll think about her criticism. That’s not a lie. You already do think about it. Just don’t give her the satisfaction of getting into an argument. The best way to stop a tug of war is to drop your end of the rope. You aren’t letting her “win”. You are putting yourself above it.
The data now suggests that we will be in some kind of quarantine for quite a while yet. It’s important to your growth and health that you find a way to distance yourself from your sister’s drama. I hope you will take some of the suggestions I’ve made.
In addition, I hope you will take care of yourself as best you can. That means getting enough sleep, eating right, finding a way to get some exercise and, if at all possible, getting outside for a bit every day. It would also be helpful to do some journaling and to learn some kind of meditation practice. Put all that on your schedule and you’ll be less available for your sister’s nonsense.
I wish you well.