You sounds like a caring brother, wanting your sister to understand rather than just doing what you feel is best for you and your family. I don't know that you can make her understand though, I would just keep with the agree to disagree commitment.

My husband does not understand my interest in fantasy and the imagination, and if he does not understand what I am saying or how I'm feeling will ask me, "Is this one of the things I won't get?" and if I say, "Yes" we just drop it there :-)

If you are having family difficulties at the moment, no need to go into them with anyone who does not seem to understand, just say you cannot make the party, you do not have to give a reason or get any one else's approval. Tell your sister you are not attending and if she keeps raising the issue, tell her you have told her your decision and she is not respecting it so unless she has other topics of conversation, you will no longer respond to her at all.