My FI and I have been together for over 4 years, and are just recently engaged. He has two younger sisters, one who is 25 and one who is 20. He is relatively close to each of them. I have always been closer to his youngest sister. She and I have similar personalities, and get along really well. His other sister is pretty much indifferent to me. When she comes over, she hangs out with him, and basically only talks to me when I ask her questions, or speak directly to her first. She has shown zero interest in being even remotely friendly with me over the past 4+ years.
His youngest sister helped him pick out the engagement ring, and was the only person who knew he was going to propose in advance. She's so excited, and really wants to be a part of everything.
I am having a small wedding party, and would like to include his youngest sister - not because she wants to be a part of everything (I'm well aware of etiquette, and am expecting nothing except that she show up with the dress on the day of the wedding) - but because I love her, and want her by my side.
My question is not should I include one sister and not the other. I want to know if anyone had similar situations, and how did you handle it? I don't think that his middle sister is going to be disappointed per se, but FI thinks that her feelings will be hurt if she's not asked. I don't want to bring it up to her, and I think it would be cruel to do so, but I know that it will be the elephant in the room regardless. Any words of wisdom or advice would be appreciated.