My porch camera caught my brother with his mistress. But he’s hiding a darker secret.

Dear Prudence is Slate’s advice column. Submit questions here.

Dear Prudence,

I had to go out of town for a work conference for a couple of days, and had my brother come over to feed and walk my dog. As it turns out, my porch camera revealed that he’s been cheating on his wife—his mistress showed up at my house! Even worse, I could see that she is visibly pregnant. As if that wasn’t bad enough, my sister-in-law is weeks away from giving birth to their first child. What the hell do I do?

—My Brother Is a Bigger Dog Than My Actual One

Dear My Brother Is a Bigger Dog,

I’m sorry for what you’re dealing with. But it’s so incredibly difficult to make a call about what should be done in these kinds of letters. I’ve said this before, and it’s still true: I wish all of our relationships came with a form similar to an advanced directive that states our wishes about situations like this. We’d have to check “yes” or “no” after, “Do you want to be told if a person who cares about you has evidence of your spouse’s infidelity?”

My thinking is always informed by the basic belief that a victim of infidelity deserves to know, complicated by questions about whether they would want to know, and muddled even further by concerns about the mental health and practical effects of upending their relationship and life. I’m especially cautious in this case because of your sister-in-law’s pregnancy. This cuts both ways because she’s in a vulnerable position, but also because the sexually transmitted infections that are a risk any time a partner is unfaithful could represent a more serious threat to her—and the baby—at this time. On top of that, I try to weigh the closeness of the relationship between the person with the bad news and the person receiving the bad news to determine whether it’s a “mind your own business” type of situation and whether the news will be believed.

With all that in mind (and without knowing the answers to some of my big questions), I think you should share the footage if your brother refuses to come clean himself, and you are prepared to go beyond dropping life-altering news on your SIL to help her deal with the aftermath. I don’t know what her needs will be—maybe she’ll need a place to stay if she decides to leave, emotional support, babysitting, or help finding an attorney. But you should pair the incriminating footage with an offering that can make the following days and months less traumatic. Because even if she really appreciates the truth, it will make her life harder before it frees her from an unfaithful spouse and ultimately improves her life. I’ll wait to hear back from you around the holidays to discuss the relationship between you and your brother, which will warrant its own whole letter.

Dear Prudence,

A few days ago, I was at the doctor’s office with my wife to discuss my upcoming same-day surgery procedure with a nurse. What happened has my wife livid, and I don’t think I did anything wrong, so asking for your opinion. Almost 30 years ago (my wife and I have been married 25 years), I dated a divorced lady, “Marie,” with three young girls. All three were named after a country, and their father was in prison, so money was tight for her. We dated for about a year, and she decided she couldn’t continue to afford to live in the area anymore and had to move back home with her parents, and I had a one-bedroom house, so no room there. I tried long-distance dating, but that didn’t work out.

At the doctor’s office, we were meeting with a nurse who had the same name as one of her kids on her name tag. I asked if it was her, and mentioned her maiden last name. She looked at me and said her last name was now “Smith” and said, “Why do you ask?” I told her that she probably didn’t remember me, but that I had dated her mom for a while, 30 years ago. She questionably looked at me, and I said I had the faded red pickup that her and her sisters called “the Barbie pickup.” She said, “Now I remember, your parents had that player piano.” She asked if I still had that (which I don’t), and that she and her sisters still talk about it on occasion. I introduced her to my wife, and a few other comments about my kids, her family, etc., were made. The conversation wasn’t more than five minutes, and we went back to discussing my surgery.

Boy, did I get chewed out by my wife when we were leaving the office about how inappropriate it was for me to bring this up at a doctor’s office and why I would even say anything to her, considering I had dated her mom 30 years ago. My wife knew about this relationship, and we even joked about whether we were going to have a theme for our kids’ names, as my ex once did with her daughters. She has been giving me the cold shoulder, minimal care in helping me after the surgery, and says I need to apologize for my behavior at the doctor’s office. I still don’t think I did anything wrong, especially when I point out that she had no problem talking to her ex-fiancé’s family when we happened to cross paths with them. According to her, “This was different.” Your thoughts, please?

—Brought Up a Past Relationship. Mistake?

Dear Brought Up a Past Relationship,

If you’d asked me before having the conversation, I might have said to hold off just because your nurse was trying to do her job, and we have no idea whether her memories of your relationship with her mom—or her mom’s dating history in general—are happy ones. But that would have been the super-cautious approach, plus you didn’t plan to see her. What you did was fine, human, and normal. The evidence of that is that your ex’s daughter engaged in a perfectly pleasant exchange with you. You didn’t do anything wrong.

But my opinion about that does nothing to help you live in harmony with your wife. Rather than digging in your heels, talk to her with the goal of learning more about what upset her. Could this be part of a bigger story about being a bit too friendly with younger women whom you two encounter as you go about your lives? Or a sensitivity about your past relationships because of a tough patch you two are in? Resentment over having to care for you after this procedure? There’s something there, and if you don’t figure out what it is now, this isn’t the last time you’ll find yourself caught off guard by her anger.

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Readers often have great suggestions for our letter writers, occasionally disagree with a point Prudie makes, or simply want to provide some additional advice. Each week, Prudie will be replying to some of these comments and suggestions from readers, which will be featured on the site on Fridays for Slate Plus members. Write to us! Or submit a question here.

Dear Prudence,

My mother has never approved of my husband (he’s not a religious nut like she is), but lately she has taken to trying to end my marriage by claiming he’s been cheating on me. Two days ago, a woman left a message on our voicemail claiming how fantastic he was in bed that past Saturday. Oh, and the woman happened to sound suspiciously like my mother.  Little did she know that my husband and I had been out of town for a funeral that day. I’ve just about had it.  Would it be reasonable at this point to tell her this stops, or I’m done with her?

—You Didn’t Fool Anyone

Dear You Didn’t Fool Anyone,

Yes.

Classic Prudie

My wife of 23 years walked out six months ago with a suitcase and the family dog. She quit her job and went to a rented mountain cabin to “find herself.” I became a single parent overnight with no forewarning—she never asked to go to marriage counseling or gave me any indication she was unhappy. I got a text and came home from work to a hysterical 15-year-old asking why her mother was leaving without a word.