Jane is right that Colin never sent the letter.
However, he did say in his statement that he didn't allow the twins to go to the farm between March and August 1985 because he was unhappy with Junes "religious mania". The Bambers might have been aware of his reasons and been afraid that he would not allow them to see the twins if he had full custody of them. That might in turn have made them keen on the idea of the boys being fostered locally. Sheila could have gone to live with them of course and taken the twins with her, but Sheila didn't want to do that, so the Bambers would know that there was a possibility of Sheila losing any custody of them.