Your sister-in-law needs to get herself a good child custody attorney. Your question really isn't a grandparents' rights question but a parents' rights one. Anyone, grandparents included, may file a petition alleging that a child is in danger and seeking custody. But parents have a fundamental constitutional right to raise their children unless they are proven to be unfit. A parent may be found to be unfit as to one child and not another due to that child's specific needs and circumstances, though leaving a younger child with her is some evidence that the grandparents aren't really concerned with her fitness as a parent.
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