Legally speaking, you have no obligation and there is no point in asking for a salary.

The father-in-law is your wife's father, and of course he is also your father. Legally speaking, a son-in-law has no obligation to support his parents-in-law, but since your parents-in-law have daughters, your children have the obligation to support their grandparents when they grow up. Reasonably speaking, a couple, if you have the ability, I think you should do your wife's duty and kill the old man.