I feel sorry for the children in the family. They have witnessed their father & uncle being cruel to their pet cats at least this once, filming it and laughing at it. They are probably either traumatise or desensitised, either way they’ll be affected by this. They may well face a backlash from other children at school, etc., and to top it all they have now lost their pet cats.
I won’t watch the video, but perhaps the child holding the cat when it was slapped was trying to comfort or protect it - I don’t know, but what a lesson about human nature!
Very very sad.