There is no demonstrable incest motif in Yarrow, though some people seem to have misunderstood the ambiguous wording of one version in particular (I forget which, but it's been discussed here, speculation and all, in the past) and read into it a meaning which really is not there. I doubt very much if there is any incestuous sub-text in The Cruel Brother, either; but that won't stop people imagining one, and of course I can't prove that there isn't, though there is no evidence that I'm aware of to suggest that there is.