Testo Opening Narration

So, did you ever hear the story of the Johnstone twins
As like each other as two new pins
Of one womb born on the self same day
How one was kept, one given away
And did you never hear how the Johnstones died, never
Knowing they shared one name
Til' the day they died,
When a mother cried My own dear sons lie slain
And did you never hear of a mother so cruel
That there's a stone in place of her heart
Then bring her on, and judge for yourselves
How she came to play this part