Give him a serious joke.

The old man and the woman have two daughters, a big son-in-law and a little son-in-law.

Burn paper for father-in-law's son-in-law-the younger son-in-law burns paper for the older son-in-law, and vice versa (otherwise you won't be called father-in-law).

Burn paper for his son-in-law's father-in-law-this old lady (the old man is dead)

So the relationship between two people should be the relationship between son-in-law and mother-in-law.