The more I've had time to think about this, the more I'm beginning to understand it and I think this reaches much further than an overly blown out of proportion fight.
Growing up, I was the oldest child of two youngest children. I am a very quiet, shy person who has always had issues standing up for myself. My sister was born with all the confidence in the world.
My sister would often get her own way, because she learned early how to manipulate. She learned that if she cried harder/longer/louder my parents would eventually give in. That extended to me. She wanted to play and I didn't? She would cry and my parents would force me to play with her.
My grandmother told me that she was down visiting one weekend and I was outside playing with a friend. In runs my sister and tells my mom to send my friend home so I would play with her. My grandmother apparently threatened my mom within an inch of her life to not send my friend home.
My aunt tells another story about how she was watching us one day and my sister wanted to play and I was reading. I told her I would play with her when I was done. Sister pitched a fit and my aunt told her to suck it up. The fit continued and my mom called in the middle of it. She asked my aunt what the issue was and when aunt told mom that sister was upset because she wanted to play and I said no, mom's response was to tell aunt to tell me to play with sister.
My other aunt, my father's oldest sibling, once went head to head with my dad over the car and curfew. A stipulation for me to get my licence was that I would drive my sister where she needed to go. Often my sister would take advantage of this. Even without her influence, I was always home well before curfew because sister had an earlier curfew than me and by the time I got her home, going back out was not worth it - I would just get to my destination and have to turn around and come home to make my own curfew. My aunt was so upset and found it so unfair that she lent me her car for the weekend and forbade me from driving sister.
So, the moral of the story? My parents ill prepared me for standing up to her. I have NO idea how to do that. And she was raised to believe that she has the right to whatever she wants. With that in mind, once I get home, I'll be looking in to some counselling to help me develop tools in which to deal with her.