Do you know anyone like this?

It sounds like she may be depressed, my father is manic depressive and has been for years, he won't look up from the TV or computer to see the kids screaming and shouting at each other around him. Being depressed would make her not care like it seems. Another thing I'm guessing is she may be burned out depending on how long she's been a parent. Don't wear yourself out taking care of your sister's children though. There comes a point in time when you need to put yourself first. Hope this helps!