@AustinTink I can tell you about my family because as I read your posts it sounded like you were describing my family. I also have a sister who was a problem in our family. Everyone walked on egg shells around her. The reason I quoted the above is she does have a personality disorder and it is Narcissism. She is extremely manipulative. As long as no one challenged her everything was ok but if you disagreed with her you would feel her rage. I was the object of most of her anger because I didn't accept her bull. I tolerated her for my mother's sake because that is who she would take it out on when I made her mad. Now that my Mother is dead, my sister, brother and I don't speak to her anymore. There was a lot of drama while my Mom was sick and my brother and older sister learned what I already knew. She was unreasonable, manipulative and a liar. Unfortunately, she has two children. I am the godmother of her daughter. Because of their mother they go along with everything she says but they are only 18 now. I don't know how they will be when they are in their 30's like your nephew but it sounds like your sister has been manipulating him a long time. I understand what a lot of posters have said but I also understand what you are saying. It sounds from your comments that your relationship with your nephew was starting to go downhill before the wedding invitations. Take it from someone who knows from experience this may be the time to cut them out of your life. It sounds like you have to walk on eggshells for both your sister and your nephew and that is not worth it. It also sounds like you have plenty of other siblings you can be close to and continue to have relationships with. When we cut my younger sister out of our family our holiday gatherings became so much more pleasant and we became so much closer. I don't believe in forgive and forget unless both parties are doing the forgiving and forgetting. In our case it was always one sided. I for one don't need that. I'd rather forget it all together.
Now to the wedding. The reason I started to read this thread is because I am going through the same thing with my older sister's son's wedding. We were not invited to their destination wedding to Disney. I was hurt but I didn't get angry. My older sister told me they could only invite so many people and that was fine. My older sister and her other children got mad for me when she found out the bride's family was inviting 15 and the groom had 4 which includes my sister. To make matters worse they didn't invite 2 of my nephew's siblings for lame reasons and then told my sister she had to pay for a rehearsal brunch which includes all 24 people that will be there. Not just the bridal party. My sister does not have that kind of money. They are trying to work it out but she is angry and my other nieces and nephews are angry. So I totally understand all sides of your dilemma and I hope things work out but I wouldn't worry so much about having a relationship with your sister or nephew based on what you have revealed here. It doesn't sound like it is worth it. My life is so much better and happier without my younger sister and family in it. I hope my experience can help you.
I am glad your house is going to be ok. I hope you recover quickly from the flu. It sounds like you need a vacation.
I am sending you some pixie dust.