Wait, but the code doesn't reference any family objects, only the family variables..
Exactly, you are grabbing the variables not from the object you want but the arbitrary family object which will have different values to that of the objects variables you want.
Like I said though I am still getting to grips with families but from my experience and testing so far it seems you cannot reference variables on the fly of child objects like you are trying to do when you are looking for the drop probability variable.
The best I have found so far is to make an array such as the following:
And for every monster assign it an NPC ID and look up the values you want in the array.