her sister and her good-for-nothing husband were as unDursleyish as it was possible to be

[diamondcutter]

Mrs. Potter was Mrs. Dursley's sister, but they hadn't met for several years; in fact, Mrs. Dursley pretended she didn't have a sister, because her sister and her good-for-nothing husband were as unDursleyish as it was possible to be.

Source: Harry Potter and the Sorcerer's Stone, J. K. Rowling

I’d like to know whether the pronoun “it” could be replaced with “they” refering the Potters, at the same time, "was" changing to "were".

 

[5jj]

No.

 

[diamondcutter]

Does the pronoun “it” refer to the situation of the Potters being unDursleyish?

 

[diamondcutter]

What about this? Is it acceptable?
Her sister and her good-for-nothing husband were as unDursleyish as they could.

 

[Tarheel]

Does the pronoun “it” refer to the situation of the Potters being unDursleyish?

It's more or less a stock phrase: as ____ as it was possible to be. In other words, you can't get any more ____ than that.

This one is obviously a made up phrase, but it follows that pattern.

 

[Tarheel]

What about this? Is it acceptable?
Her sister and her good-for-nothing husband were as unDursleyish as they could possibly be.

OK.