According to my country's culture, it's not his responsibility to take care of his mother-in-law unless he is willing to do so. Even a wife can't demand it except requesting it. If an in-law demands something like that here, it's called shameless and such kind of situaion most relationships are bound to be broken.
0.00000000 BEE
It is common in Nigeria though, amidst the hungry in-law, we have the disciplined one that don't go around asking their son in law for stuff...
It means some culture need to be changed.