Use 'on any touch start' instead of 'on touched boxes.' That will be more accurate, since the event will run even if the user misses a box by a little bit.
Give the family a variable, 'selected.' Then all objects in the family have that variable. Have events like this:
On any touch start
Pick nearest boxes to touch.x, touch.y
If distance(touch.x, touch.y, boxes.x, boxes.y) is less than whatever amount
- set family variable 'selected' to 1
If boxes variable 'selected' = 1
- set boxes position to touch.x, touch.y
On touch 0 end
- set boxes variable 'selected' to 0
If you're not sure how picking works, I suggest going through the starter tutorials if you haven't already.