Interesting slant on this. There is much more of the 'impossibilities' theme in these variants, but it's easy to see where in some British versions the 'sun and the moon meeting in the sky' come from. I think 'The Cruel Brother' 'Son David' and 'Lucy Wan' are all closely related and all derive from Scandinavian versions. 'Lucy Wan' I think, (only a hunch, I can't prove it) is a fairly recent development of the theme, probably 18th century.