Alan of Australia, my version of 'Riddles Wisely Expounded' (Child #1) has the same burden as Pentangle's 'Cruel Sister', but the explanation I have for it is quite different:

'Lay the bent to the bonny broom' (a phrase of 'physiological significance' - 'bent' = 'horn' - says Miss Margaret Dean-Smith who has a sharp sense for euphemism). (A.L. Lloyd, 'Folk Song in England',p 153f.)

'The erotic implications are emphasized by the first refrain line, for 'broom' here means the female private parts.' (Roy Palmer (Ed.), Everyman's Book of British Ballads 198)

Susanne