I was surprised by the slap, as well. But then, it was probably an instinctive reaction - Sahil kept on chanting how happy he will be if his Bua died. Yes, the Bua is a horrible person but for Anika, she is still Sahil's Bua - and you shouldn't wish for anyone's death like that, especially not little kids. Anika understands that and that she has to teach Sahil these things; the slap most likely only happened due to the things that transpired beforehand and stressed her out, I guess.
But I am sure Sahil will forgive her. How long will he stay upset with his Didi? đ