As stated already, this Officer was doing his duty. I am sure the Paramedic would not have allowed her to perform a breathaliser test if it was deemed she was unfit. The OP states his sister was hysterical which is understandable but no reason to not submit to a test. Clearly the sister was not so injured as to render her incapable, as she made a phone call to her brother.
As always, there are 3 sides to every story. His side, the officers side and the truth. Here we are being told the side of a person who feels wronged at being asked to step away by an Officer because he was obstructing him doing his duty.
Lesson learned - do as you are told so that everyone can get their job done effectively and efficiently.