The key words are "Any complete round".
A round for sisters is you do platforming, dunk a sister, you damage a sister for 1 minute / kill ads. That part ALWAYS repeats. Just because you can't damage both sisters doesn't mean it's a round.
If you kill each sister in 1 round, that's still 2 cycles.
If you damage sis 1, kill sis 2, kill sis 1, that's 3 cycles because you've gone through 3 COMPLETE rounds. The rounds are complete because a round is platforming, dunk, damage/ads. That's the definition of a round. A sister doesn't have to die to be a round.
In a boxing match, there are typically 12 rounds for the main event. You dont have to knockout the person for that to be a complete round.