Your sister can start training as a counsellor without having any experience at all.
She can start with the "Basic Counselling Skills" course, one three hour session for one term. From that she can get a place on the "Certificate in Counselling" course, one day a week for one year. Completing this will get her a place on the "Diploma in Counselling" course,one day a week for two years plus, I think it is, either forty or sixty hours in couselling.
What I have described here is, what used to be, the main route into counselling. These courses still exist but they are not the only ones nowadays. Check out your local college. Many colleges provide courses of training up to professional level.
On completion of these courses, plus having the required number of hours counselling, she will be able to join the British Association for Counselling and Pychotherapy (BACP) She will then be able to apply for most counselling positions If you look at job ads for counsellors you will see that they, generally, ask for the "Diploma in Counselling" and BACP membership. This is their website:
http://www.bacp.co.uk/
Your sister would also, of course, be able to start her own counselling practice. She could do this now if she wanted to. There are no regulations governing counselling. Anyone can "hang out their shingle" with or without experience or qualifications. I am not suggesting she does this, however. I mention it only to demonstrate why employers value BACP membership. It is a reputable organisation and has strict standardsof membership.
It isn't necessary to have a degree in psychology but it doesn't do any harm. If your sister does intend to do one, tell her to make sure it has the "Graduate Basis for Registration (GBR) with the British Psychological Society. If she want to become a chartered counselling psychologist she will need this.