Supervised and unsupervised

Looking to see AITA? (Am I the asshole?)
So my twin and 8yr old nephew(Adhd diagnosed)are over earlier (they are here like half the week we are super close) we talked about going outside and she said she didn’t want to and to go alone, fast forward 5 minutes and she says she is gonna have a shower, I ask my 3yr old if she wants to go for a walk to the store and then go tell my 10yr old and her 8yr old that we are going to the store and would be right back store is a 2minute walk from house. So my toddler and I head out, my son comes running out as he says he wants the fresh air, and were already on the way out so we go and come back 5 minutes later and my sister is livid because her 8yr old decided to go outside in the back yard which is fenced and my landlords and their son are out backand he hurt himself, not badly at all. But she is mad because I didn’t inform her I left. She is claiming I left her 8yr old child home alone, mind you her son has to pass her in the bathroom to get outside. And she is claiming I must apologize for leaving her child home alone!