My 8 year old daughter is hateful and rude to her twin sister.  She also mocks her sister every chance she gets.  She also shows no affection towards her sister.  This saddens me as I would love for my daughters to get along.  I have repetitively asked her to be kind to her sister and I have punished her by taking away privileges to get her to stop.  Unfortunately, nothing seems to work.  What should I do now??