Is this something you feel only in relation to your sister or in other situations, too? If it's just your sister, then I think you guys should work on it. I'm an only child, but I see with my dh and his brother that some of their modes of relating to each other go back so far that nobody knows how they got started and some of them are so tied up with longstanding family mythology that their nearly impossible to untangle.
Is your sister the older one, or the "smarter" one (I mean as far as the family mythology goes, iykwim), or the one who's always right?
type I diabetic and mama to ds1 (9/14/03) and ds2 (2/11/09)
sometimes vlc, sometimes pure carnivore, always once-a-day feeding