Замечу, что в оригинале про родственника-бухгалтера сказано вот что:

'Are all your family wizards?’ asked Harry, who found Ron just as interesting as Ron found him.

‘Er – yes, I think so,’ said Ron. ‘I think Mum’s got a second cousin who’s an accountant, but we never talk about him.'

То есть ни "маггл", ни "сквиб" не упомянуты.