I and my wife are planning to invite my wife's parents this summer, my mother-in-law is a housewife and never had a job, and no bank account as she is completely dependent on my father-in-law financially as most elderly couple in India are, can we submit my father-in-laws' bank statement and employment proof with my mother-in-law's application as well ? since she doesn't have any financial documents of her own?
This is an extremely common situation.
All your mother-in-law needs to do is say that all costs will be paid by your father-in-law, and attach your father-in-law's financial statements as evidence that he can take care of it. I believe that if you apply together you may not even have to attach the statements twice. In terms of income and expenditure your mother-in-law should indicate zero income and expenditure (or whatever small amount of independent income she might have, if any) and your father-in-law the total expenses of both of them.
See the linked answer for more details about regular income and expenditure.