Dear Jitendar...
first point Dispensary have no rights to ask or demand you to get sub code number...as opting any dispensary for both individual and family members is the discretionary power of Insured Person;
Second point Sub Code number is a number taken for any branch office/sales office/ admn office etc., when they are in a different location other than the main unit...now a days its so liberal that maximum of 999 number of sub codes can be taken in one state.. But in your case we cant expect employer to take several sub codes for each employees family purpose, the main purpose of sub code is not for medical purpose, its for the purpose of cash benefits of the employees who are working far away from the main unit, like sales men etc., but for getting a sub code number existing of any office is also not mandatory.
In case of any problem u r facing with the dispensary which u want to get treatment for your parents, you can explain the same at the dispensary or even u can resolve the issue by talkling to the concerned ESIC regional office by approaching the branch office (local office) concerned...Even now a days all ESIC institutions are linked with VoIP number, ie., they can contact any of the offices through out india for free of cost.... So make use of it and get your problem solved.
I wish you all the best...