Setting clear boundaries in these situations can be crucial. Like actively/preemptively deciding what behaviors are acceptable, and consistently enforcing those limits. It’s also important to keep in mind that your sister’s behaviors may often stem from insecurity; not that it should excuse her actions, but understanding that NPD can act as a defense mechanism for low self-worth might aid in managing interactions with her more effectively. As you know, dealing with a narcissist can be quite exhausting, so be sure to prioritize your own mental health- empty cups, etc. But ofc, keep the children’s well-being in mind- maintaining your own self respect and dignity is important but, at the end of the day, there are kids involved. Best of luck to your family- I hope things work out as best they can for everyone involved!