Most likely sister has used the cost and/or time commitment involved in activities with her daughter as excuses for not helping. I have a relative who hid behind his kids as an excuse for not even visiting his grandmother 20 minutes away. For years. No one asked him to do anything but walk into her room and talk to her for 10 or 15 minutes a few times a year. Nope, too busy with kid stuff.

Yes, again, as I've said throughout this thread, it is the sister who is the problem, not the niece. But if she's constantly citing her daughter as an excuse it is not surprising that a tired and frustrated caregiver is mistakenly directing anger at the person being used as the reason she doesn't get help.