Am not a supporter of the RSPCA, frankly, but this is such a difficult call to make. If the RSPCA had refused to take these cats, and their owner hadn't made a recovery, they'd have been criticized for that. Damned if they do..... damned if they don't.....
The sister has to bear some of the responsibility in this situation, though doubtless she felt she was acting for the best. Very sad about the cat which was PTS