Near the end of Season One of 'Kingdom', it was stated that Peter Kingdom's father Roger was a notorious shagger in his day. He probably stepped out on both his wives (the second one being the mother of Peter's siblings Simon and Beatrice).
It could be that Roger Kingdom had no qualms in chasing after married women, some of whom may have become pregnant by him and then forced to pass off the resultant offspring as their husbands' children.
If so, Peter Kingdom may have at least one half-sibling he may never know about - Ros Myers, who works for 'MI5' in 'Spooks'.
Ros grew up believing that her father was really Sir Jocelyn Myers, a well-to-do diplomat who was involved in the conspiracy to force the Prime Minister to sign a special measure act. This would have allowed the co-conspirators (led by MI6 head Michael Collingwood) to lock up people without trial and to use the police to prevent civilian protests.
It was Ros who helped expose the conspiracy and saved the lives of Harry Pearce and Adam Carter in the process. She severed her ties to MI6 afterwards and joined MI5 where she has risen to become the head of MI5's Section D after Harry Pearce.
She looks a lot like her half-sister Beatrice Kingdom, but probably doesn't have the same sort of mental health issues......