Marriage certification issue for my parents

[kap]

Hello, 

 

I am living in USA from last 14+ years and on Greencard. I will get my citizen ship in next 2 years. Once, I get citizenship, I would like to call my parents to USA. 

I understand that birth certificate and marriage certificate are must to file for citizenship. 

However, for my case my parents have lived all their life in live-in relationship and do not have marriage certificate. 

What are the options I have to call my parents here and they can stay here for rest of their life. 

I will be visiting my home town in next few months and can ask to prepare necessary documents if needed. 

Please reply, this is critical question for me to proceed.

 

Thanks