beta
Sher reads Kas a fairytale about the witch. In the fairytale it states the witch bets a wager of some sort and if the human wins she will grant them a wish for anything and if they lose she will take their soul. It continues that even though the bets are always in favor of the human the witch has never loss because a soul "full of love" makes her more powerful and she refuses to lose out on that. In the middle of the story Kas, states he must be really powerful since he loves Sher (his mother) so much, and if that isn't a foreshadowing I don't know what is.
If I had to pin point, I think the witch knew that Sher's whole family had the potential to be full of love, but that none of them currently were. Sher only thought of her mother, and wasn't a fan of her husband and son so there wasn't any love there, and because the husband and son weren't loved by their wife and mom there wasn't a "full of love" soul within them either. But if she could send Sher back in time and eliminate the reason why Sher was having such a hard time accepting the love of the people around, she could get get them to be a loving family, and take the soul that she wanted who was the most filled with love. Which I would assume would be Kassel based on chap 28.
This is just my hypothesis though~