Hi, Starx. Sorry you are having this problem. I know it must cause you much hurt, embarrassment, and anger. Most of us would feel that way, and it's hard not to take it personally. For me, dealing with an 'emotional attack' like that would mean changing my response.
Since you can't change your sister, the only thing you can do is change the way you react. We could spend a lifetime analyzing why she feels the need to do this, but that would be a waste of effort. What you CAN do is to tell her she could be right about you. Tell her you agree that you don't have the courage she thinks you ought to have.
Tell her (and tell yourself) that people who are short on one trait are often excellent at another. Blind people have extra-special hearing. People who flunk math often have the best reading and writing skills. People who don't do well at sports frequently succeed as scientists or engineers. And so forth.
Be careful that you don't attack her with hurtful words. Don't say anything at all about her traits or skills. Keep the focus on yourself. And lastly, tell her something like this: "when you say that, I feel hurt and embarrassed". Then it's up to her to decide if she wants to keep on hurting you.
Best of luck to you as you deal with this issue.