what are the ways to ruin an interview?

Well without discussing with your sister what she said or did in the interview, it's impossible for me to know what she did wrong, unless of course you're asking us to do guesswork here? As a HR professional I have conducted many job interviews over the years and obviously have attended many interviews too. I can usually tell from the interviewers body language and tone how well or not I have done in an interview. Some of the things your sister might have done wrong could be: 1) She was too nervous and stuttered throughout the interview - interviewers should be prepared for this and it's also the interviewers duty to try to make the candidate feel at ease to avoid this nervousness on their part. 2) She was asked a specific question or questions about her skills relating to what she on her CV/resume and she answered incorrectly or made it seem as if she had lied on her CV. 3) When asked if she could provide examples of what she would do in certain situations, she might have got confused and given a totally irrelevant answer. 4) When asked if she had any questions for the interviewer at the end of the interview, she might have said she had no questions at all, thereby seeming not interested in whether or not she was selected. I have written articles about interviews and how to cope with them on helium, if you'd like to have a look or refer your sister to them, you can find them here: http://www.helium.com/tm/439296/things-really-during-interview1chew http://www.helium.com/tm/180879/ensure-youre-rested-interviewif http://www.helium.com/tm/169833/think-toughest-questions-people