system
Given the following post, respond as if you're commenting on Reddit:
human
If I know my father is cheating, should I tell my mother?

This was a moral dilemma which came up in my literature class as we were discussing "Death of a Salesman". I understand the consequential argument that telling the mother would only cause her pain and destroy the marriage so telling her would be immoral. However, for some reason I can't get over the feeling that withholding this information from her would be wrong. Would it be selfish of me to tell my mother? Is there a moral argument for telling her (perhaps deontological or virtue) or is it just a mental hump I have to get over?
gpt
There is certainly a deontological argument for telling your mother (and a virtue ethics one as well, and we could even make a utilitarian argument: just think about how much better it would be for your mother to hear from you, now, than to find out at a much later date and realize that you knew all along). I’ll focus on a Kantian argument for now though. 

For Kant, it’s vitally important that we respect the autonomy of others, especially with regard to crucial, life-altering decisions. That means that we should trust their reason and their decision-making capacities. Your mother has a choice about whether she will stay in this marriage, address her husband head-on, or some other course of action. It’s not your place to prevent her from making that decision by withholding the vital evidence (or by failing to make her aware of the potential problem). You ought to treat her as a rational agent, like yourself, not an emotionally loaded powderkeg who might not be able to handle the truth. Treating her humanity (her rationality) as an end requires giving her the option to exercise it, so the Categorical Imperative requires that you not withhold such information. (Of course, you ought to find an appropriate private setting to tell her, you shouldn’t overeagerly jump to conclusions with insufficient evidence and overdramatize things, but you still have a duty not to hide your realization from her.) It is not selfish of you to tell your mother (unless you only do it for selfish motives); it is responding appropriately to how invested she is in her marriage, how much of a stake this information has in her life, and her right as a rational agent to determine her own course of action for herself, in light of the best available evidence. 

As a side note, the virtue ethics argument would be something like: honesty is a virtue, and so is compassion. If you’re sure that the father is cheating, it’s virtuous to make your mother aware, but (again) in a considerate and thoughtful way, without overstating your case or jumping to conclusions.

(And as the other commenter notes, it’s certainly not a bad idea to discuss this with the father too. But that doesn’t negate the importance of making the mother aware.)