I do think the framing has to do in large part with the LW's situation: The question isn't really "Is BIL's behavior ethical?", it's "Should I tell my sister about this?", and the answer to that is yes regardless.

But I agree with you that the couple's behavior is potentially really, really skeevy, especially depending on how much they know about BIL's financial situation – and especially because it sounds like BIL may have turned them down the first time they asked. And it might have been good for Prudie to push back a little on the "affair" framing.