The OP makes a good point.
The intention was to destabilise and create drama, and it worked. But who's to say the Twins, Harriet and Sarah wouldn't be at war with Marc and Simon by now, which would itself be good drama.
Then again, the Twins were the most feisty, so would probably have been evicted.
The best result would have been for father-of-the-house, Kieran to go, as he was smoothing over too many cracks, and allow Harriet, Sarah and the Twins to stay and argue with the newbies.
Allowing Sarah to stay would also have fortified Danny to fight for his place in the house more strongly, as now he has nothing and noone to care about, and just sits about being useless.