Your brother in law is an idiot?
Or, your brother in law is an idiot.
You managed to upgrade all seven of you and he flushes that down the toilet?
I agree with jd2000; perhaps he's worried about behavior problems. Maybe he thinks he'll be asked to move his family back to Y mid-flight if the kids act up (or simply act like kids)?
Perhaps he's suffering some sort of "pride" and is embarrassed that you're able to so easily move him and his family to F and "doesn't want your charity."
Perhaps he's afraid of spoiling the kids (and knowing he'll not likely ever be able to upgrade his family, doesn't want them experiencing it)?
Or maybe he's just an idiot. So is my brother in law.