If the "bent" is a coarse grass then the laying close to "bonnie broom" would be a metaphor for the corrupting power of sexual desire, cf. "Let no man steal your thyme": "A woman is a branchy tree/And man's a clinging vine". Besides, "lay" is rarely used without a sexual connotation.

I don't see the grass being the wicked sister; in fact, one could argue that she is the broom which has been choked by the grass.

Sorry to be so "one-track", but all the old folksy songs were re-vamped in the 60s with a distinctly sexual flavour and full of menacing double-entendre (cf. Jack Orion: "He neither kissed her when he came/nor when from her he did go/and in and out/of her window..." etc.)