The Cruel Brother (child #11) is not about incest. But it makes sense that we would come to that conclusion today because the moral code of asking each member of the family for permission to marry has died away. So we search for some other reason her brother would have killed her. See "traditional tunes of the child ballads" by Bronson,

"I grant little credence to Barry's notion (BFSSNE, VII 1934 p. 8) of a veiled incest-motive behind the murder, not so much because of the repugnance of the idea as because the 'folk' would have felt little need to veil it, if that had been the point, and formerly would have found the motive sufficient without it" -(V. 1, p. 185)