You can file a Petition for Guardianship. That way you can make "parental" decisions and cover her with medical insurance.
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File a guardianship petition. You will need the names and addresses of your niece's parents, siblings and grandparents (the court MIGHT let you continue without some of those names and addresses. )
For a more immediate solution, have an attorney help you get a caregiver's affidavit which can be done quick and cheap and which a insurer should honor and allow you to purchase insurance for your niece without a guardianship.
I agree with counsel, you should fle a petition for guardianship.
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