The Licensing Act 2003 removed the statutory requirement for bar staff to be 18 years old.
The situation in pubs is now similar to that in supermarkets. If you purchase alcohol at a check-out staffed by someone who is under 18, that employee has to seek permission from the licence holder (or, more usually, from any other adult employee to whom the responsibility for authorising sales has been delegated) before making that specific sale. The store manager can't give 'blanket permission' for a minor to serve alcohol. Every single transaction must be separately authorised.
So, if you employ your sister she must ask you (or another authorised adult) for permission every time she sells alcohol. (To comply with the legislation, the person giving permission must satisfy themself that the purchaser is not under-18, or already intoxicated, or otherwise prohibited from purchasing alcohol. If they don't do this their permission will not be valid). On a busy Saturday night, that means that your sister may have to seek permission hundreds of times.
Chris