I think I saw a version on DT that had the fa la la... refrain. I also heard a recording by Pentangle (John Renbourne, Jacqui McShee, et al.) with that chorus, but it wasn't a cappella. Here's my transcription of their lyrics:

CRUEL SISTER

There lived a lady by the North Sea shore
Lay the bend to the bonny broom
Two daughters were the babes she bore
Fa la-la-la-la la la la la-la

As one grew bright as is the sun
So coal black grew the elder one

A knight came riding to the lady's door
He travelled far to be their wooer

He courted one with gloves and rings
But he loved the other above all things

Oh, sister, will you go with me
To watch the ships sail on the sea

She took her sister by the hand
And led her down to the North Sea strand

And as they stood on the windy shore
The dark girl threw her sister o'er

Sometimes she sank; sometimes she swam
Crying sister reach to me your hand

Oh, sister, sister let me live
And all that's mine I'll surely give

Your own true love that I'll have and more
But thou shalt never come ashore

And there she floated like a swan
The salt sea bore her body on

Two minstrels walked along the strand
And saw the maiden float to land

They've made a harp of her breastbone
Whose sound would melt a heart of stone

They took three locks of her yellow hair
And with them strung the harp so rare

They went into her father's hall
To play the harp before them all

But as they laid it on a stone
The harp began to play alone

The first string sang a doleful sound
The bright haired younger sister drowned

The second string as that they tried
In terror sits the black haired bride

The third string sang beneath their bow
And surely now her tears will flow

Hope this helps.

HTML line breaks added. --JoeClone, 5-May-02.