Sisters or brothers voices always seem to go together well don't they.

Descant type harmonies seem to be their favourite manouvre. (A harmony above a tune - often, but not always, a third above the melody.

In Complainte pour Ste Catherine they use at least 3 and probably four part harmonies. (one above, and two below, the tune.)

The La la la bit at the end is definitely four part, tune on top, simple ascending and descending movement in the (sung) chords

If you could be more specific NIck, (i.e. name a song) I'll figure out what makes it tick.

MofA