and if you were in a place where being mistreated by your sister was something you could ignore or effectively put a stop to while still being in their lives, I would say that you were being selfish

When you say "ignore", do you mean "avoid contact/abuse", as in having the option to stay away from her & just seeing the kids? (which we know isn't possible) Or do you mean still being exposed to her, but just withstanding her mistreatment (which would inevitably mean stuffing my emotions down & also imply I matter less than my family)?

He'll remember his Tia who loved him

You don't think he'll be too young to have many memories? He's only seven after all.