Archu and Sachin had a very good, loving relationship at 1st, and Savita started to tell him how he will be neglected by Archu and Archu doesnt love him cause he is not her son and she will only ever favour Soham. She brain washed that child from a young age. AND neither Manav or Damo did anything to stop it.
So... u really can't be mad at Sachin... he was very young and impressionable when Savita messed with his head.
Just my opinion and 2 cents from how i remembered it.