Yes, you sound right, sleepinoz. I was thinking of it in terms of the comments that Karen and Sarah were close; and given that Karen was not a biological mother to Anthony Rickard, then it would leave Sarah as the only child of Karen. I think that is the context in which Sarah said she is an only child.
I am quite surprised people are making a lot of this. Given that Anthony was estranged from the family, and given that Sarah is Karen's only child, i don't think Sarah's comment was odd at all; in fact, it was correct, she is an only child with reference to Karen, and the radio interview in which Sarah made the comment was regarding her mother. Sarah could have said, mum, you left me and Anthony behind. LOL Somehow i think that is beyond the pale, given all the events that have happened.