How do I perform an action only on an objct of a family?

for example: on mouse click I change the position of the object I clicked on and not on the other object of the family.

There you go.

EdwardjT92's example works even if your condition is "family clicked" and action is "set family position to 0,0" it will still only move the clicked object since the click "picks" that member only. If your trying to do something less selective you will need to make sure of which family member you are "picking" in the event conditions. You can do this by using distance to something (ie. closest instance, etc), UID, or using instance variables inside family members.. IsChoosen=0... IsChoosen=1...

what if I want the action to be repeatable? click on an a member of a family and affect only that member, then repeat the action on another member and affect only this other member and so on?

It already is repeatable, although edward didn't use a family in his example, but I guess you noticed that.