Good day Peonyrose73, sorry to hear you are going through such a difficult time.  I also have a sister, and felt that I was the “invisible” one, being she had humor and a much bigger personality than I did/do.  Now as I am older, I feel much better with the situation, as I am happy with who I am, and realize my sister has her own issues, as everyone does.   Personally, I would go to the wedding.  I know your sister said she didn’t care if you attended, but deep down, I bet she does.  Even if you went for the main portion, whilst telling yourself that you can leave whenever you want.  As you can.  Can I ask how old you are?  Are you on your own now?  I have to say that things got better for me when I moved out and found my own friends (like family).  Hopefully you will experience something similar, or just being around those that build you up is important.  Keep me/us posted, and I wish you peace.  x