Originally posted by steverules
I thought he had known his children had lived, judging by the fact that in ROTJ he reveals he knows about Obi-Wan hiding Luke and Leia from him.
He only said that because he had just found out that Leia is his daughter by reading Luke's mind. Add to the fact that he knew that Obi-wan had been watching over Luke since he was a baby would lead Vader to realise that he was also hiding Leia as well.