Tough one, although I agree with everyone saying that ultimately, it is your (hypothetical) parents' money to do with as they please. Emotional reality is not always as clear cut as that, though.
When we were younger, my parents used to give a LOT more money to my sister #1, because she asked for more. I always made do with what they gave me... and yes, when I discovered the discrepancy, I was resentful, because my sister and I were living in exactly the same conditions, she just had less reluctance than me to ask for extra money to upgrade her life. My parents didn't give her a lot more money, mind you, but it still bugged me that I was dressed in ratty old sweaters and unable to go out with friends while her clothes were closer to average and she went out a few times a month. I still didn't ask. It didn't spoil her, the amounts were not insane, and she is now a completely responsible adult.
Frankly? Looking back on that one, the onus is on me. If I was bitter about my standards of living, and unable to get a job, I should either have mentioned it to my parents, or I should have accepted my pride in not taking handouts as its own reward. Wanting not to have to ask and yet for people to somehow feel compelled to hand me undeserved money was quite immature.
Today, it is to sister #2 that my parents are giving a LOT more money, and I don't mind at all; in fact, I am happy they can help her, simply because she has just not been as lucky in her financial life as I have. If part of her bad luck was the bad luck of being stupid with money (not the case), it wouldn't change a thing. I would expect my parents to try and do the best for her--teach and protect her as is necessary.
The only case I can see being resentful, then, would be if my parents for no other discernible reason than preferring her would give way more money to one sister. It would still be their choice to make, but emotionally it'd be painful, because I haven't gotten to the level of wisdom where it wouldn't hurt to feel the "least loved" of my parents' children. If they were clumsily pouring money trying to help my sibling live "on par" with me, on the other hand, I'd think they were mistaken in their belief that bailouts would solve anything, but I would not feel hurt.
As for the "lost" money, you can't lose money you never had, so whatever.