Who is more entitled, Islamically, to serve elderly and sick parents: the married son (by forcing his wife to care for them) or the married daughter who lives with her husband? And what is the ruling on the service of the son, his wife, the daughter, and her husband to the parents?

Serving parents when they are in need is obligatory upon children, both male and female. If they have no one but a married son and a married daughter, then it is incumbent upon them both to care for their parents and serve them, either directly or by hiring a servant for them. As for the son's wife and the daughter's husband, it is not obligatory upon them to serve the husband's or wife's parents, but whoever volunteers to do so will receive reward.