I think it's fine to tell someone they've hurt you but it's also important to hear people when they tell you that you've hurt them. I would go to a councelor and or therapist and have my sanity validated. When everyone around you is mad at you or shrugging their shoulders and walking away from you, it couldn't hurt.
Forgiving youself is important.
No matter how sorry we are, sometimes if the consequence of our actions hurts someone else, our forgiving ourselves realy has no effect on the damage done to them.
Here is an example. My 50 year old friend was recurrently sexually abused by her alcoholic father starting at the age of five. She still goes to counceling once a week and her mother thinks she just does this as a means of unforgiveness. Her mother is very sorry she didn't intervene, this many years later, her mother has forgiven herself and gets angry that my friend won't let this go.
Your reasons to go to see your niece are valid and your reasons for not going are valid. It is your choice to go or not. It's a busy time, graduation and I think your sister was just warning you, as you should have really expected, if you are looking for exclusive time with her, this isn't a good time for that.
What is your daughter mad about, what happened? We can't gauge her unforgiveness without knowing what happened.
It never hurts to have a periodic third party opinion on occassion.
What I hear you saying is that you feel a bit taken for granted, a bit victimized by unreasonable loved ones and a bit hurt that you are judged but not forgiven.