It seemed super weird to me that two, but only two, of the kids would be removed from custody just because the teenager committed a crime, even if the crime was against another kid in the family, especially if the accused was no longer living there

If the teenager sexually abused a sibling, and the parents were all la la la head in the sand about it, and refused to cooperate with authorities about making sure it never happened again,

I can definitely see that resulting in child removal.