I feel like Shiika doesn't really know, probably her memory of her dad dying got blocked that's why she's acting innocently. I mean, I don't think it's believable how relaxed she is. The story will probably progress to her remembering those memories and I wonder how she'll react then.
I agree, Shiho's character has been written beautifully. At first, I thought she was a random character just to cause drama, but I'm really invested in this arc. The buildup to her character is good, I don't really get why they're hating on her so much.