You can receive temporary custody or temporary guardianship through a court order. Your changes of receiving this without your nephew's parents' cooperation are slim to none. If you want to keep your nephew because it's in his best interest and his parents are allowing that to happen, you can continue that indefinitely. If you want his child support, you can ask your sister for it. If she doesn't give it to you she needs it more than you do.
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The longer you can keep him(as is)-the greater your chances with the court later.
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
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