I imagine that Ivar says "who cares?" because he has very little empathy or reason to care about anybody else. Siggy was a young child who apparently drowned in the river, which would imply that she wasn't taken care of to begin with you. In the scene you are talking about, Aslaug appears to be both very bored in playing the game with Ivar, and also drunk. When she's informed by Sigurd that Siggy was found dead in the river, she actually struggles to remember who the little girl is, and then goes onto say "I thought somebody was watching her." I don't know the whole story of Ivar's upbringing, but from what I have seen on the show, he showed the traits of becoming a monster even as a child. He was born a crippled person in a time when physical strength was highly prized, and defined a lot of masculinity. He was told by his own mother that he should be grateful that he wasn't left to die in the woods at birth. His father wasn't around very much, and the only one who seemed to show an interest in him was Aslaug, and she constantly babied him and shielded him from any consequences of his actions. If you go back a bit, you'll find a scene where Ivar puts an ax through the head of another child over a game. Aslaug promptly sweeps him up and coddles him, completely ignoring the killing. I can't really find any example of Ivar being taught anything like empathy, or regard for others, so it would make sense that he would ask "who cares?"