demand the school schedule a in person conference between you and the parents do the other child. While this could me a simple childish threat, it should be address a serious by both sets of parents and both kids so they both understand why it okay to say things like that. You would be able to keep an eye on them and they would be much more safe with you. You wouldn’t be running two households and wouldn’t be stretched thin. You could install security cameras to keep an eye on them if you had to go out. You would know they were eating healthy food or even eating. It would keep you from doing a lot of running and it would give them a better quality of life if they are struggling alone.
My parents always made a big deal out of being fair. If my sister got some extra pocket money or a new piece of clothes, so did I. They were meticulous with that kind of fairness. The fairness that you can measure. The favoritism that hurt me was emotional. My mothers emotional responses to me and my sister were so different, that it still puzzles me to this day, that she couldn’t see it herself. Or maybe she justified it to herself with the fact that my sister and I were very different people with different needs. Which is true, but even so, that stands as another tragic evidence of how little she was able to see and understand me.