Depends on your twinship. I'm a fraternal twin and my brother and I basically hated each other.in part because he was taught that being mean to me could earn him our mother's favor, in part becauae we were lower middle class trying to make ends meet and except for clothes and shoes we were supposed to share everything. We'd get presents for "the twins" and I think there are several times we nearly sliced something in half rather than share it. My brother was also very popular at school And certainly above average intelligence but no genius, whereas I was basically too smart for every school I was in til my very good high school and kind of a nerd so he thought I was bad for his image. We didn't have boundaries so much as a giant wall with a drawbridge we let down once or twice a year.