1. I'm presuming your parents have not traveled to any other visa-required countries, right?
2. Are your siblings married?
3. Do your parents have bank accounts and/or any financial investments (even locked-in FDs, stocks/shares, jewelry as an investment)?
4. Can one of your siblings financially pay for your parents' visit? Your finances are secondary for their applications
5. Do your parents volunteer for any social, charity or religious reasons?
6. Do your parents receive pension?
7. They must file their ITRs even if it is below the taxable limit
8. How long do your parents intend to visit? Purpose of their visit?