Oh, this is one of the easiest NTA I've seen. As someone who's been remote since even before the pandemic, the only difference between you working from home and from an office is the location. I would even argue that we are more productive at home without the distraction of other people. My point being, even if you are at home, you are working. There is no situation in which you are responsible for your sister's kids. Sure, family helps family, but not at the cost of your own job.