[dalke]

As the story goes: I and a friend were walking down the street. A robber came up and demanded all of our money. I pulled $80 out my wallet, looked over to my friend and said, "oh, here's the $40 I owe you" while passing it over. The robber became so mad over this grave injustice, and demanded that my friend loan me back the $40 plus the rest of my friend's money. The robber then stole the lump sum from me. I now owe my friend $150!

In any case, I think you're missing part of the equation. Value isn't a stand-alone concept. It isn't "the value we place on a dollar" but in how much each of you values family meal time. Your youngest sister, at that time, placed a low value on it, compared to other possibilities that she could do with the money.