this is a guess because I can't see the notice. the IRS probably denied the dependency exemption for your sister because she is too old - likely over 24. however, the IRS is wrong. Age is irrelevant if she is totally and permanently disabled. however, the IRS in its lack of wisdom provides no place on your tax return to indicate this.
what you need to do is get a signed and dated letter from her doctor stating that she is totally and permanently disabled. then you send back the notice with your signed and dated letter saying that your sister SSN xxx-xx-xxxx is your qualifying child because she is: 1) RELATED. 2) HER AGE DOES NOT MATTER BECAUSE SHE IS TOTALLY AND PERMANENTLY DISABLED PER THE DOCTOR'S LETTER INCLUDED (IRC 152(c)(3)(B) 3) SHE LIVED WITH US FOR MORE THAN 1/2 THE YEAR 4) WE PROVIDE OVER 1/2 HER SUPPORT 5) SHE DOES NOT FILE A JOINT RETURN
PLEASE ADJUST YOUR RECORDS ACCORDINGLY PER OUR RETURN AS ORIGINALLY FILED.
if she is not over 24, then you can use the Turbotax tax notice section to review the letter to determine the reason for the adjustment
https://support.turbotax.intuit.com/irs-notice/audit-support/
