That is very disturbing... You should see the look on my face...
Behavior like that, without any remorse, is a huge red flag. That's how serial killers start.. It shows compulsive behavior, a damaged concious, and the ability to be violent without feeling guilty over it. You will see this show up when she interacts with other children, will probably start getting in trouble at school, will begin to become more socially reserved, etc.
Sounds like little 8 year old NEEDS a counselor. NEEDS. The 15 year old? A good talking to because her attitude needs to change. Just because she's a teenage girl doesn't allow her to be all-mighty brat with a "I don't care about nuffin!" attitude. She needs to gain some respect and responsibility.
I swear to God, if someone messed with my animals, this would not fly. Animals are the most innocent creatures, what will they do to humans, who are not innocent at all?