Familt visitor - Medical Insurance

I am a British citizen and planning to apply for a family visitor visa (for 3 weeks) for my father and sister-in-law in India.

My father had already been (and returned) to UK once. He is over 65 years of age - do we have to mandatorily include a medical insurance in the supporting documents?

I will be fully sponsoring their trip. Would following documents suffice:
- My passport Photocopy
- My invitation letter
- My bank statement
- Their passports
- Their bank statements (my fathers account doesnt have much money)
- My sis-in-law employer letter + property doc + marraige certificate

Would this be enough to convince visa officials that they will be returning to India? Is their any other document needed?