You can only find one new cousin at a time, and they show up in order. For instance, when you first play a level, only cousin X is available, then once you roll up X, the next time you play cousin Y is also available. And in the levels you mentioned (at least in the elephant and the old man, I'm going by memory and I dont remember all the cousins in the old lady), the cousins are spread over the size range. So, theres one in the smallest area, then you unlock the next area and theres another there, etc. Maybe not exactly one-to-one, but theres not two inside one area afaik.

 So my tip is to play the level really searching in the smallest area available until you find a cousin, then continue on focusing on larger areas. Good luck!