NTA

If a child did that to my pet and their parent 1) failed to discipline to prevent this; 2) did not apologize; 3) INSULTED ME - I would retort, "I love children, just not undisciplined children and their horrible parents" and let them know they would not be welcome back in my home until she apologized and could raise something other than a little animal abusing savage.

NTA, they are both brats.

How are you out of line for protecting your child? It doesn't matter if you don't have children - if anything, it makes it worse.

Part of parenting children is raising to respect others' homes and other animals/people/children. She failed.

How DARE she. This isn't even about parenting. It's about drawing boundaries in your own home, which children can trespass.

Your sister is an asshole.