Buffy Sainte-Marie's "Incest Song" has elements of other ballads, most notably "Sheath & Knife" (Child 16) and "Lizzie Wan (Child 51). Here's my transcription of what I hear on Spotify.

THE INCEST SONG
(Buffy Sainte-Marie)

Word is up to the king's dear daughter
And word is spreading all over the land
That's she's been betrayed by her own dear brother
That he has chosen another fair hand

Many a man had the song of her beauty
And many a grand deed for her had been done
But within her sights she carried the child
Of her father's youngest, fairest son

Tell to me no lies
Tell to me no stories
But saddle my good horse and I'll go and see my own true love
If your words be true ones, then that will mean the end of me

Brother oh brother what lies be these ones
They say your love to another I lose
There's a child within me of thy very own lineage
And I know it's I that thou wouldst choose

And have you yet told your father or mother
All that thou hast told here to me
And he's taken off his good braided sword
That's a-hangin' down beside his knee

No I've not told no one but you my dear one
For it's a secret between us two
And I would come home and quit all my roaming
And spend my days only waiting on you

Too late too late for change my sister
My father has chosen another fair bride
And he stabbed her easy and lovingly lay her
Down in her grave by the green wood side

And when he's come home to his own wedding feasting
And his father asks why he's weeping all so
He says such a bride as I've seen on this morning
Never another man shall know


https://www.youtube.com/watch?v=r_IHdCI3uuU