Interview question?

Pls house, what do interviewers seek to find out when they ask you how many siblings you have. And like how old is your younger ones and the year they are born. What has your family position got 2 do with job seeking and position. Pls your response will highly be appreciated. I faced that question in an interview and that seems 2 be what failed me when i told them we are three and cant really remember the age of my younger one.
Most importantly, i think it is not my job but wants 2 know the proper way 2 tackle such question.