The Transportation Security Administration (US)’s website is useful to show what they do for passenger security screening. They recommend (but it is not a requirement) that you bring documentation to verify your sister’s condition. This will very likely help speed up the screening process at any airport anywhere:

http://www.tsa.gov/travelers/airtravel/assistant/editorial_1049.shtm

I suggest your sister approach her doctor or her specialist to ask if they could provide a confirmatory letter stating her condition. If she has any medication, she must bring it in the original bottle with the stated medical information on it. Unidentified pills in plain unlabelled containers may slow the security process down too.

Please note that the TSA allows airport operators to opt out and contract out their own security staff. Therefore they also this guideline document which outlines the various requirements for their screeners including training and what they should be able to do – if you are interested in more details:

http://www.tsa.gov/assets/pdf/SectionJ_07_14.pdf