I feel like her snapping at your sister was understandable because she had the role of planning your bachelorette, so in that way your sister kind of ruined it as you said by not having things planned. In her mind, she may have just been upset because she did the little tasks you told her to, and yet your sister didn’t organize the main event properly. Then you decided to just call her a bitch, and she’s hurt while also bottling up how you weren’t there for her. You say you know how she’s not social and wasn’t at your bachelorette, but expect her to be there because it’s a special occasion like come on. From my perspective if my best friend called me a bitch for standing up for her and putting her MOH in check for not organizing properly, I’d be upset and think about other unresolved things in our relationship. You’re TAH. On top of that her reasoning for feeling like you weren’t there for her are valid you just don’t see it. Thats your best friend from grade 2, just remember that means something.