“Surely the lack of that extra money is a comparative disadvantage?”
“Advantage” and “disadvantage” are not really what it is all about, are they? Happiness is not about how much you create compared with how much you might have created under some different circumstances; it is about whether you are successful in solving the problems that you actually have.
Thinking in terms of comparing a given family with and without a multi-million dollar inheritance is not to the point, because it does not bear on any decision that that family actually faces. What is to the point is the usual, real-life situation in which the family’s wealth does depend on decisions that they make, and in which the relevant values ought to be integrated (not weighed!) with other values in the light of various considerations.
If a family manages to do that successfully, then it is not meaningful to ask whether they would be better off richer, because they are as rich as they have decided to be. Not only are they not suffering from any lingering impulse to decide differently, they have created a problem situation for themselves where their success in the respects they have chosen to value, and hence their happiness, depend very little on whether they receive an unexpected inheritance or not.
Sarah Fitz-Claridge, Taking Children Seriously FAQ: ‘Surely the lack of that extra money is a comparative disadvantage?’, https://www.takingchildrenseriously.com/surely-the-lack-of-that-extra-money-is-a-comparative-disadvantage/