Am I morally obligated to tell? That's my simple question.
What is your basis of morality?
Religion: I don't think religion obligates you to tell his wife. Religion does ask you to stop him, however. Religion also asks you not to spread the words about vice.
Health: Definitely, people need to know what risks they are getting exposed to.
Otherwise (social, cultural): No obligation at all. It is your choice to tell or not.
My opinion will not change in case of my or anyone's else sister as well. My opinion will also not change if a friend's wife was doing this sort of thing.