How old is your sister? You cannot get the child tax credit for anyone over the age of 16, even if they are disabled. There are no exceptions to that age limit. If your sister is your dependent and you have to pay someone to take care of her so you can work, you can get the child and dependent care credit---which might be what you have confused with the child tax credit. The child and dependent care credit is non-refundable. It can only reduce tax you owe. It is not given to you as a refund.
https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-f...
https://ttlc.intuit.com/questions/1900643-what-is-the-child-and-dependent-care-credit
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