The prettiest tune has to be the one recorded by Pentangle, but I like this one for group singing. As usual, if there is a third sister, she is only mentioned in the first verse. This one is from Hindman, Kentucky. I like the bits about the beaver hat and the mermaid/swan.

The Two Sisters

There lived an old man by the northern sea
Bow down
There lived an old man by the northern sea
The boughs they bend to me
There lived an old man by the northern sea
I will be true, true to my love
Love and my love will be true to me

A young knight came a courting there
And he made choice of the youngest fair

He gave the youngest a beaver hat
The eldest she thought much on that

He gave the youngest a gay gold ring
He gave the eldest not one single thing

Oh sister, oh sister let us walk out
To see the ships go sailing about

But when they reached the north sea brim
The eldest pushed the youngest in

Oh sister, oh sister lend me your hand
And you shall have my house and land

I'll neither lend thee hand nor glove
But I will have your own true love

Sometimes she sank, sometimes she swam
Til she floated into the miller's dam

Oh miller, oh miller go draw your dam
Here's either a mermaid or a swan

It's neither a fish nor a swan I see
But the water hath drowned a gay lady

The miller came out with his fish hook
And fished the fair maid out of the brook

He robbed her of her gay gold ring
And then into the water he pushed her again

The miller was hung at his mill gate
The eldest sister was burned at the stake