I don't want to go to my sister's graduation
That's sad. Why don't you want to go?
Because I'm a pathetic man who really wants her life to be miserable.
You don't seem to care about your sister. She is family and that should be respected. You would want her to visit your wedding or other important live events in the future, don't you? Life is short and you don't want the people in your life to grow smaller and smaller.
I think she is too good for me, which is the reason why I have to do it.
She is part of your family, no family member is too good for you. If you really do not want to go, try to give her a strong reason why not, so that you still would have reached out instead of ghosting your sister.