I just wanted to write and thank you, and your readers, for your great advice about my "special snowflake" sister/MOH and the +1 dilemma. I did what you suggested (pulled my head out of my ass and invited the boyfriend). My mother told me that she thinks I'm a bitch who needs therapy "for putting my sister through such trouble" and the grandparents were less-than-thrilled, but my sister was happy, and all was well. I felt better, sister felt better, things were good.
Then he got sick and missed the wedding and reception anyway.
Anyway, thanks for your advice. It put my mind at ease knowing that I made my sister feel better, and I needed input from somebody outside my immediate circle.
All's Well That Ends Well
Sophie Vlaming by Klaas Jan Kliphuis via café au lait