Has my sister's drunken behaviour gone too far for reconciliation (long, sorry!)?

My younger sister (29) is getting married this summer and me (32) and my older sister (42) are going to be bridesmaids (we are both already married). We are all close but the other night something happened to change this. We all went out at the weekend on the first girls' night out we have had together for about a year as me and my older sister both had babies last year. We had a great night and at one point, a bunch of guys a lot younger than us came up and started chatting. We made it clear that we were all taken as soon as we met them but one of them still took a bit of a shine to me and followed me around the club like a puppy. NOTHING happened at all and I stayed with my sisters all night, just dancing and having a great time with the girls. Then, when we got back to my younger sister's house where we were staying the night, I had just put on my PJ's ready for bed when my older sister (who'd had one too many to drink) just completely turned on me. She said she was very 'disappointed' in me for not being more ertive and telling that guy to "f" off. Everything was fine though, the guy hadn't been a problem at all so both my younger sis and I insisted that nothing had happened so why was there an issue? My older sis then called me all the names under the sun and then started on my younger sis, calling her a "little s***." I was in tears by this point and my younger sis was defending me to the end, but then my older sis locked herself and my younger sis in a bedroom and I overheard my older sis saying "You don't know what she's like - something happened with her and your fiance five years ago - he made a p at her!" Now this is utter rubbish and the first I have ever heard of it. Then my little sis went screaming downstairs to her fiance demanding to know what happened and he, of course, denied it too but older sis was insisting that it had happened because our mum had told her. This led to both my sisters physically fighting on the stairs and older sis locking everyone in the house and pushing us all out of the way of the door. We were all in tears, my younger sis was fighting with her fiance and while I was screaming my innocence, my older sis was shouting abuse at me and calling me a liar. I honestly have no idea where all this came from at all. It went on for a good three hours and we were on the verge of calling the police. Then I got my mum out of bed to intervene. She came over at 4.30am and she said she had never said such a thing to my older sister, which proved my innocence. It was an absolutely horrendous incident and now, three days later, I am still in a trance and emotionally scarred by the experience. My younger sis is covered in bruises from the fighting and she has just about made up with her fiance. The next day, I expected my older sis to be begging forgiveness on her knees but I wasn't prepared to talk to her when she came knocking on my door hours later the next day and so she sent me about ten abusive text messages justifying her actions and still calling me a liar. Even my husband didn't know what to think about me after hearing from me about this 'guy' and then my sister's fiance. Now, what I would like some advice about is, should my older sister still be bridesmaid at my younger sister's wedding? If you were me, could you bring yourself to plan the hen night, wedding etc with her and do so many bonding things when this awful thing has happened between you? Right now, I have said that I will not be bridesmaid if she is, but what should I do? I never want to see her face again but should I put this aside for the sake of my younger sis? My younger sis is hoping that we can put all this to rest for the sake of the wedding but should she still want my older sis as a bridesmaid after what she's done and if so, is that not really disloyal to me? Help me out and I'm sorry to ramble on for so long!