Real Sister Dynamic?

I have one brother and he is my best friend forever.  We were the bestman in each other's weddings.  He gave his son (who didn't live outside the womb) my middle name in honor of me.  I love my brother.  And my brother has become the only brother my husband has known too; when the three of us are together the joking, the craziness are nonstop.

I really cannot relate to the terrible things two of the three sisters do to each other in Downton Abbey.  If my brother was in trouble, then I'd help him - not harm him.  A win for my brother is a win for me.  I cannot be jealous of my brother's achievements; I celebrate my brother's success.  However, the sisters in Downtown Abbey are jealous, competitive, and cruel. (Edith wrote the letter to the Turkish Embassy, and Mary sabotaged what would have been a proposal to Edith from her boyfriend, etc.)

Do you think that's true to life?  I can't imagine being this way towards my brother, so it's hard for me to believe they could be this way towards each other.  If you're a sister and you have a sister and you've watched Downton Abbey, then please let me know if you think this sister dynamic is true to life?  Is it possible for sisters to try an destroy each other?