T1, E1: This first episode sees the return of Tia, Tyler, Theo, Lola and Taylor - now 5 years old - as well as great friends, Alfie and Emily, who are now 6. As before, scientists are on hand to observe their every move. Why is it that siblings are one moment the best of friends and then the worst of enemies? And how will the older brother and sisters respond to meeting up with their old friends.