if it was a shop's car park, they are entitled to set whatever rules they want about who can park where, but if they are going to fine you they must display them i would have thought. You say it didn't say "blue badge holders only" are you sure? In my local retail park for example a sign says "disabled parking" but by the spaces there is a blurb about how you must have a badge otherwise you are getting clamped and fined. It might be worth you going back to see what the signs actually say, because if it's not displayed you may be able to contest it.
If its a council run car park for the shops then it will most likely be displayed at the entrance or on a board, but again, check it out.
In my local "big" tesco, they don't really do anything about non badge-holders parking in the slots allocated, unless you go in and tell them, and then they'll put a tannoy announcement on for the person to come and move, but they cant fine them because its a courtesy not to park there, rather than an "offence"
I can only echo what the others have said that your sister can apply for a badge even if she dosent drive - contact your local council for the application,
And for everyone's infor, i read the other day on bbc.co.uk that they are changing the criteria for getting a badge - some temporary disabilities might start applying soon.