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I am sorry to hear about this situation. Without going through the court, basically the only option is a power of attorney from you to your sister. See HERE. With such a power of attorney, she would be his de facto guardian. However at any time, the POA may be voided by the actual parent (i.e. you). The POA lasts for 12 months at the most (CO 15-14-105), so it would have to be re-signed every year.
There are draw backs to this. For example, if the child ever gets into legal trouble, you would still be considered the parent/guardian and have duties thereof. You would also be civilly liable for up to $3,500 for his shoplifting or willful/malicious property damage or personal injury, or, $250 for negligence or willful misconduct while driving a motor vehicle. You may also be held responsible if your sister does not provide a stable environment for the child and the child is found to be lacking basic necessities. There is no way to get rid of this duty/liability unless custody is formally changed via the court to the sister.
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