About 170 viewers said it should not have been shown before the 9pm watershed, but the Independent Television Commission said the scene did not breach rules.
A spokesman for the programme said yesterday the sensitive story line had been responsibly handled.
'This is a very dark plot for The Bill but there is a lot more that has yet to be unravelled.
'We believe that at the end of this story line viewers will agree with us that we have treated it responsibly and in an appropriate manner.
'At the end of the day it is about one character, June Ackland, being manipulated by another one with a shady past.
'But this is a long burner which will run into the New Year and has many facets with a lot of twists and turns.'
It is not the first time soaps have used incest to boost ratings.
In 1996, Channel 4 received hundreds of complaints after Brookside characters Georgia, played by Helen Grace, and her brother Nat, played by John Sandford, were seen sharing a bed.
And in 2001, EastEnders was accused of toying with an incest storyline when it showed Steve Owen, portrayed by Martin Kemp, being given a passionate kiss on the lips by his dying mother Barbara, played by Sheila Hancock.
The incest episode of The Bill will be screened on ITV1 on Wednesday at 8pm.
