Upvote:1
While your mother-in-law is not employed income of her own, her husband's income supports the entire household and is shared. Respond to the questions in terms of total expenditures, regardless of how it is funded: food, utilities, mortgage/rent, insurance etc., everything that is spent each month to support and maintain them (and anyone else in the household). That joint/shared income against expenditures and bank statements are evidence needed to demonstrate financial viability.
It would be your father-in-law's documents that support her application: his salary slips, bank statements, employment letter, etc.