I drew a line with a mark a little bit in from the left labelled 'Sis'. Another mark a bit further over to the right labelled 'Ben'. At the right hand dot of the line, a mark labelled 'Dad' and little further to the left of 'Dad' a mark labelled 'Mum'.

As an

*aide memoire* I put '?' under the section of line to the left of 'Sis', '3' between 'Sis' and 'Ben' and '4' between 'Mum' and 'Dad'. I then did 'hops' along the line to indicate that 'Ben' sits a third of the way from the left hand end to 'Mum' and 'Sis' a quarter of the way from that end to 'Dad'.

Having drawn the pretty picture to help visualise the problem, I just played around with ages for Ben, which when multiplied by 3 give an age for Mum four years less than the age for Dad (3 years less than Ben, ×4). It turns out that the siblings are just about the same ages as DD and DS2 and Mum and Dad are 8 and 5 years younger than myself and DH respectively

Possibly rather too long-winded for exam conditions, but it may help in transferring the solving of the problem to algebra, if that's what you want him to use.