I’m so sorry you are living with this pressure every day. I’m very concerned that your family has always felt sorry for you. I wonder if it’s possible that you have an undiagnosed and untreated learning disability that has made it difficult for you and that has made you the object of pity. You didn’t need pity. You needed help. If you can, I suggest you get an evaluation from someone who is qualified to identify learning problems. Once you know what you are dealing with, you can develop strategies to better manage your life.
As for your sister: She sounds very insecure to me. The way she feels good about herself is to feel like she is better than you. I wonder why she doesn’t feel good enough just for being herself. Is it possible that your parents’ pity for you meant that she felt she wasn’t getting enough attention? Whatever the reason, I hope she gets the help she needs so she can stop feeling good only by acting superior to you.
You are both in your 20s now. Your big sister can only have power over you if you let her. Being human, I’m sure she isn’t perfect in every way. You don’t need to accept her judgment. You don’t need to hide from her. For your own self-respect, you do need to find a way to stand up for yourself.
One of my teachers used to say that the best way to handle a critical person is to “take the sail out of their wind”. The best way to do that is to not argue with her and not let her see that she is affecting you. Thank her for her concern and let her know that you are doing your best to take care of yourself. Then go about your business. This is your time to focus on making whatever changes you need to make to feel better about yourself.
I wish you well.