A bit belated, but thanks to all who responded. I've done alot of self work about all of this, trying to figure out what I needed to do and what I needed from Joe and Sue to be happy. It was becoming difficult because even when I was trying to be patient and understanding with Sue and trying to work with the "she's just socially awkward, not malicious" thing, if I asked any questions about her behavior (in polite ways), Joe immediately got defensive, which I found really insulting because I have never been mean to or about her and have always gone out of my way to be considerate of her. It was creating a them vs me dynamic, and I'm definitely not having that! Luckily, one of our D/s things is that I write a daily journal and Joe reads it once a week. Once he was able to read how unsupported by him I felt and how I wasn't trying to judge Sue or fight with her, but that I just felt constantly threatened by her presence because I didn't know if she would be hurtful again, he seemed to get it more. He was able to understand that I need to see a pattern of non-hostile SEEMING behavior on her part- I don't care about excuses, by age 30, unless you have a disorder, you can learn a few basic social skills and implement them regularly. (And I'm saying this as an introvert myself- I just had no choice but to learn how to deal with social stuff coming from a huge family and going in to performing arts.) I made it very clear that I needed to see some personal responsibility on her part...and if the pattern of hurtful behavior continued, I would not be able to remain in a relationship with him, as much as that would hurt me. That REALLY seemed to make things click for him!
I'm not sure what has been discussed between him and her, but we recently all spent New Year's Eve together. Before the day arrived, Joe asked me several times to come up with ways he could better support me and help make it comfortable for me, which I appreciated. I treated myself to a spa day the day before to get calm and refreshed, and before going over did restorative yoga and meditated, so I was able to go in there feeling very calm and positive. Sue behaved nicely- she even said hi when I came in! (Small blessings, right?) It wasn't the easiest night ever...but it wasn't the worst either. I'm trying to go forward taking baby steps with her and knowing I have to set the boundaries that work for me. I'd like to be as friendly as possible, that's just my nature-but I also need to not let her lack of social skills cause havoc on my own happiness or my relationship with Joe.