Is it something that is possible with Gravity Forms:
Using a drop down, to select the number of siblings somebody has, eg: 1,2,3,4 or 5+
Then make depending on the response, other fields appear.
Say if they selected 5, 5 groups of 2 text boxes, asking them for name, age of each (and all are required as they selected 5).
If they selected 1, only 1 group of 2 text boxes would appear (and be required). etc
Thanks.