Like many old folk songs there are many variations on the lyrics and chords out there 
being played and sung, so this may not match perfectly what you know – hopefully it's 
still useful.

For some reason my mother always sang this in Bb minor (capoed up 1), but put it 
wherever your range is. Chords repeat from verse to verse.

       C       G         Am
Three sisters walked out one fine day,

Am                   C     Dm
All the lee and the lonely–o,

      Am   (G     Am)
They met a robber on the way,

        C              G           E7   Am
On the bonnie, bonnie banks of the Virgio.

He took the first one by the hand,
All the lee and the lonely–o,
He whipped her 'round and he made her stand,
On the bonnie, bonnie banks of the Virgio.

Oh, will you be a robber's wife?
All the lee and the lonely–o,
Or will you die by my pen–knife,
On the bonnie, bonnie banks of the Virgio.

Oh, I'll not be a robber's wife,
All the lee and the lonely–o.
I'd rather die by your pen–knife,
On the bonnie, bonnie banks of the Virgio.

And so he took his wee pen–knife,
All the lee and the lonely–o,
And there he took her own dear life,
On the bonnie, bonnie banks of the Virgio.

(– Repeat for the second sister. –)

(– Then, repeat for the third, until her answer, which is below. –)

Oh, I'll not be a robber's wife,
All the lee and the lonely–o,
And I'll not die by your pen–knife.
On the bonnie, bonnie banks of the Virgio.

Oh if my brothers had been here
All the lee and the lonely–o,
You wouldna' killed my sisters dear,
On the bonnie, bonnie banks of the Virgio.

Where are your brothers pray do tell
All the lee and the lonely–o,
One of them's a minister
On the bonnie, bonnie banks of the Virgio.

And where is the other one pray do tell
All the lee and the lonely–o,
He's out a robbing like yourself
On the bonnie, bonnie banks of the Virgio.

Oh, my God, what have I done,
All the lee and the lonely–o,
I've killed my sisters, all but one,
On the bonnie, bonnie banks of the Virgio.

And so he took his wee pen–knife,
All the lee and the lonely–o,
And there he took his own dear life,
On the bonnie, bonnie banks of the Virgio.