This particular set of parents seem more interested in justifying what they did than in making any effort to mend it, and they don't seem to recognize that unequal financial treatment does damage that isn't financial. The sense I get from the letter is that the writer is really looking hard for a loophole that will make them not owe the apology that they clearly owe.

I wonder about situations where siblings get vastly, unfairly different amounts of money support from parents for reasons that aren't the parents' fault, though. "When our first was born, we were really just scraping by, but by the time the youngest got here, we were more stable," or, on the other hand, "When our first was born, we were in a great position, but by the time the youngest got here, that bubble had burst."