It used to be very common when there were people had several children. Usually they'd be spaced out over about 10 or 15 years so if the eldest had a child relatively young, they could well only be a couple of years younger than their youngest aunt or uncle.

There is actually the same sort of age gap between Max and Bianca as Ian and Bianca (about 8 years) and only 4 between her and Jack.

A colleg2 friend of mine is 8 years younger than her nearest sibling and only 3 years older than her eldest niece so she was more brought up with her nieces and nephews than her sisters - odd as that might sound.