Almost impossible to enforce such a rule. Consider, for example, the case of an unmarried couple.
Yes, it would have to be based on Ts & Cs (eg: children, parents, spouse, partner, etc). I would think threat of the dreaded FR would keep people in line, but I know there's no good solution.
99 AU was clearly ridiculous, but 2 seems a bit weird (if you want to cut kids/parents/etc out, why not limit it to 1 AU?)