I already had this situation and found a workaround. You can do it with "pick by UID". You have a first event that picks an instance of FamilyA. Then you have a sub event that picks an instance of FamilyB by UID, providing the previously picked FamilyA member. Since it's supposed to be the same object, the UID is also the same. You can now access both FamilyA and familyB instance variable, which should refer to the same instance.
+ mouse over Family A
+ pick FamilyB by UID (FamilyA.UID) --> Set FamilyA instance variable A
--> Set FamilyB instance variable B
I know this is a workaround and it might not be as elegant as having a specific event for that, but it does the work.