Sorry S1mon I'm at work so I can't check it now...
If you have two variables; one to define the entire instance of all combined objects, and a second one to define the individual parts of each instance...
So the first instance that is created, have a var called something like "UniqueID" and the second instance called "PartID" or "PositionID"or something like that... when you spawn each part for one particular instance, the unique ID would be the same (IE, all would be 1 for the first instance, but each PART would be 1,2,3,4,etc).
THEN you can compare the UniqueID (for the entire instance) and each Part/PositionID...
For each "thing"
-> UniqueID = UniqueID
----> Set position to PartID imagepoint.
In this way you have a "global" identity for which parts belong to which instance, as well as the positional identifier so it knows where to join on to the instance.
Kinds wish I wasn't at work... I've done this so many times with making several instances of the same "blank" character that use multiple parts... in almost every prototype or project I've worked on, I have a single "blank" that is instanced and used to make as many players or enemies as I want - all without using families or containers.
Speaking of containers though... you COULD easily use a container to create all arts at once, and easily reference all parts at once since they will automatically belong to the other objects in the same container. (Look up containers in the construct 2 manual) 8)