Britain’s Claire Douglas has a problem many mystery writers only wish they had.

As her new book “The Wrong Sister” goes on sale Tuesday, she reigns as the Sunday Times Number One bestselling author of 10 standalone novels with 2 million-plus copies sold.

Does that mean with each new book there’s a bit of anxiety about what her fans will think?

“It definitely does make me feel a bit nervous, because you don’t want to disappoint your readers if they really enjoyed the last book,” Douglas said on the phone from her home in Bath. “I try not to think about it too much, because I worry that if I think about it too much, I won’t actually finish the book.

“So I just try to write what I think they would enjoy, which is twists with characters they can care about.”

The two siblings in “The Wrong Sister” are Tasha and her older sister Alice who resemble twins. Both are married but with completely different lives. Tasha and her husband Aaron have two kids and stayed in their hometown.

Alice, a top notch scientist, is a world traveler with an equally successful husband. When Tasha and Aaron need a break, Alice and her husband come watch their kids and have them relax in their palatial Venetian digs.

That idyllic getaway instantly evaporates when they learn that Alice was attacked and her husband killed in their home. Tasha returns confused and in shock, when a note addressed to her arrives. It says: “It was supposed to be you.”

“The idea for ‘The Wrong Sister’ came from speaking to a friend of mine who had a very similar female weekend away. Luckily, that didn’t end in murder,” Douglas, 51, noted. “But it was quite interesting, having these two sisters who can look quite similar.

“I do always like to have a twist at the end, one that I can’t really talk about too much without giving it away. And I knew I wanted to have this whole mistaken identity switch-up. Apart from that, I didn’t really have much of a plot planned out.

“What I do is normally start writing it. Then it comes to me as I write, which I find interesting, because it gives me a bit of excitement when I sit in bed every day. Waiting to think what can happen.”

Her success was hardly inevitable. “It wasn’t until I had my first publishing contract — I was 39 by the time I got my first book published. That’s when I started to realize I could do it as a full-time job. And that it could actually be my full-time job.

“That’s when I realized my dream has come true.”