Query action on the after profile update trigger could work. You'd use conditionals to compare var1 and var3 values to ensure the field actually changed. Example as follows.
[var1_FIELD_NAME] Not Equal To [var3_FIELD_NAME]
Then you'd update all field values of profiles from the old to the new query a query. Example as follows.
Code:
UPDATE `#__comprofiler` SET `FIELD_NAME` = '[var1_FIELD_NAME]' WHERE `FIELD_NAME` = '[var3_FIELD_NAME]' AND `user_id` != '[user_id]'
Better option would be to just not use those fields if the account isn't head of household. Instead you could query for those values using CB Query Field or try substituting them in using the Layout parameters for those fields or a Custom HTML field. Everything depends on how you've set up the relationship between head of household and other family members though.