These were children young enough that they needed babysitting. First and foremost, these matters should be left to their parents, and you went around them to do this. YDI for that alone. Then we go to the age-appropriateness of this. Videos on animal treatment in slaughter houses from most pro-vegan sites, in addition to being often inaccurate or often decades out of date, are also just plain not age-appropriate for children and can be as traumatizing as any body-horror; I wouldn't show them documentaries of even life-saving surgeries for the same reason. YDI it again for that one. Your sister is right to go no-contact with you until you get it through your head what your role is regarding raising her children. For the record, the answer to that is "I don't have a role in raising other people's children."