Hello. I can see that while you care for your parents, helping them understand your sexual orientation is extremely important right now, considering both the pressures for marriage as well as the blame for their ill health. This news might be difficult for them to process. A family therapist/ counsellor might help you break the news and handle the consequences in a safe atmosphere.
Next Steps
You could also reach out to anyone in your current family/ friend circle who you think would be able to help you in asserting your viewpoint to your parents.You could reach out to a Counselling psychologist who will be able to help you through this. There are also a couple of helplines/ organisations in Chennai (ICWO, SAATHI, Orinam) that might be able to help ease this process for you. Wishing you the best.
Health Tips
Breaking the news in a planned way- slowly but affirmatively will help you. Be cognizant of your support system for emotional and physical safety.