Seriously, I don't think there's an answer to this. I think whoever set the question in the first place has messed up on their research (or done it from faulty memory without checking at all - it happens all too often, unfortunately) and there is NO American brother and sister who have both had UK number 1s. (I suppose if you were to count fictional characters, then technically you could say that Bart and Lisa Simpson were both on the Simpsons' "Do The Bartman", but even then, the question says number 1s, plural, so that doesn't work either).