You do not need to be referred by a doctor but may have to prove your disability through either a letter from your GP or sometimes they will organise a visit. Surprisingly they are very strict, and sometimes even if you are in receipt of a disability benefit it does not guarantee you will get a badge.
you apply thro your local blue badge office
( mine is piccadilly manchester) and fill out the form
and some sort of documentation is required
you overcome the reams of administrators who say things like
I dont think lymphoma is a cancer
if you say you are under the Christie, it doesnt mean you are
and - lets see if the sight is as bad as you say it is as a result of a secondary behind the eyeball.
and you get one - and golly it is worth all the hassle
including: is this you in the photograph ?
and being video'd - and luckily I was already walking into dustbins and telegraph poles before I realised I was on camera ( I looked around anxiously to see if anyone had seen me walk into a post box)
The late husband of my dear friend had several heart attacks, transplants etc could hardly walk without getting out of breath, but he never was able to get a Blue Badge and he died when his heart finally gave up a few years ago.
He wouldn't appeal though against the decision and it's always worth doing that.
If your sister gets turned down, tell her to appeal.