Marian is great at asking evil questions like that because she asks another sort of evil question: "Why did you expect that this Brad person would be friends with Mike just because you thought he would? A mean kid is going to stay mean no matter what Mike does." This habit of Marian's of asking terrible questions that ruin people's lives is, of course, because like most outsiders, Marian is hampered in her quest to get Elly to think straight by another Foob Fact:
Foob Fact Forty-Five: As a general rule, the Pattersons tend to not anticipate the normal consequences of an action and prefer to live in a world where everything might result in either an insane and implausible catastrophe or boundless success.
This living in a world wherein everything that's trying to kill them had a baby with everything that wants to because they hate how wonderful the Pattersons are is the reason for something that baffles and disturbs most people who have to interact with them. As you know from my comments about the Declining Years, someone unfamiliar with the family would reel with horror when confronted with something that looks like the deck of the Flying Dutchman. To the untrained eye, we seem to see a group of phantoms that pass through one another without interacting; this resolves into the troubling real-life image of something that's a family in name only. Most people by now have grown used to the fact that the Pattersons look and act like a group of strangers to one another who happen to share a house and a last name without quite realizing that their fear of a simple comment leading to the Apocalypse is what keeps them from really bonding as people.