Then, the obligation to maintain these relations is in proportion to the share which the maintainer would inherit on their death. In other words, the obligation is of the presumptive heir in proportion to his presumptive right of inheritance.
When some of the presumptive heirs are not in a position to maintain the relations, then the other presumptive heirs must provide maintenance in proportion to their presumptive rights.
Ordinarily, a Muslim has no obligation to maintain his relations by affinity. Thus, a Muslim has no obligation to maintain his widowed daughter-in-law.
But it seems that a Muslim has an obligation to maintain the wife of his son, if the son is too young and has neither the means nor the ability to earn. Similarly, a Muslim has an obligation to maintain his stepmother, if she is weak and infirm, and without means to maintain herself. But it seems that it is more in the nature of a moral than a legal obligation.