The clincher here, and every commenter didn’t even mention it, is money. 

The white sister in Texas gets ICWA money to take custody of these Native kids. And its not peanuts. It amounts to thousands of dollars per month until they’re of legal age.

I know of a Native grandparent who lost custody of his grandchildren because the non-Native “mother,” who wasn’t even around when they were born, got custody through one lie after another, and received more than $3,000 a month per child, (there were two), for almost 20-years.That's what she was after and by gawd, she got it.

The grandfather hasn’t seen them since.  That’s been a State of Alaska practice forever. ICWA isn’t worth its salt up here.