Thinking of it financially changes the possible meaning of telling his sister-in-law. It might be a terrible idea for the brother-in-law to be secretly doing dangerous, stigmatized, illegal work because he needs money so badly. But it's not an affair. (Unless of course it is partly an affair? Is the couple paying for his beauty and acting skill and energy, or the knowledge that they're bribing him to commit adultery? People are complicated.)

I know there are more exciting plotbunnies being considered below, but the ones that leap to my mind are more old-fashioned.
"Please don't tell M I had to do this terrible thing just to pay rent and buy baby food for our poor children. If she knew, it would ruin Christmas." Then M's sister [or mom or best friend] either comes through with some kind of day-saving Christmas present.