Is there any medical or psychological reason behind sibling rivalry?
Yes people behave toward their sisters or brothers in the way you described above. For no apparent reason my aunt hates her sister from the early age. I think it has to do with a hateful event that happened to the person when his/her sibling is born (time around birth or before) so they connect this with their sibling and blame him/her from the early age. Also from the astrological point some zodiac signs do not bond well with other signs, this maybe a reason too.