The couple, who share an eight-year-old daughter, gave a joint interview to The Sunday Times, in which Karen revealed: "I'm a fixer. I'm hugely overconfident and I probably underestimated the complexities of being a stepparent [to Kitty, 29, Eliza and Amelia, 28, Louis, 26, Edmund, 16, and Lara, 14].
"Charles gave me the best advice, which is to say nothing and do nothing about anything, ever."
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She continued: "I don't get to have an opinion about his children. I can have an opinion and give it to him, in our bedroom, but not to them. I just wish I could say I'd adhered to it the whole time. Stepparenting plays to Charles's strengths and my weaknesses…
"Charles's older four were pretty much grown up when I came on the scene, so my job, which I hope I've achieved, was simply to make everyone feel welcome."
Charles and Karen attend Prince Harry and Meghan Markle's royal wedding
Karen went on to admit that her daughter Charlotte is "pretty much an only child, but she adores her big brothers and sisters". The philanthropist and founder of Whole Child charity is also a mum to her elder daughters Emma, 22, and Kate, 19, who "adore" their stepfather Charles.
The Earl, who is currently promoting his new book The White Ship, married Karen in June 2011 at Althorp, the Spencers' family estate in Northamptonshire. Charles, 56, was previously married to Victoria Lockwood, with whom he shares four children, and second wife Caroline Freud, with whom he has two children.
The Earl with his older sister Princess Diana
In the interview, the author also opened up about his later sister Diana. The siblings were incredibly close and Charles gave his youngest child Charlotte the middle name, Diana. He admitted that their mother Frances wasn't "cut out for maternity", saying: "Our father was a quiet and constant source of love, but our mother wasn't cut out for maternity. Not her fault, she couldn't do it.
"While she was packing her stuff to leave, she promised Diana [then aged five] she'd come back to see her. Diana used to wait on the doorstep for her, but she never came."
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