Can I refuse to go to my sisters college graduation?
Are you serious? Is there a particular reason you think you shouldn't go to your sister's graduation?
Yes, the reason is if I go there, she will take advantage of my presence at her graduation to show off.
You may think that your sister is trying to take advantage of you to get attention from others, but not going to her graduation just to avoid this is inconsiderate of her feelings. Go to her graduation and be there for her in her hour of accomplishment.
She missed my father's funeral, so she deserves it.
It is wrong for her to not go to your father's funeral but it doesn't make it right to hurt by ignoring what she accomplished. She will be hurt by this and your relationship might not recover.