We were faced with a similar situation with my husband's brother's 2 children: the 12-year-old boy is easier to deal with than the 9-year-old girl, but it created a few tense moments when we asked that BIL keep daughter at home with him(he is a single dad, for the most part). Money was an issue, too, as BIL has come to expect us to pay for everything all the time(gotta love family) and I truly felt my neice would ruin it for everyone else, as behavior is a problem with her. Even though it is not her fault, she acts very inappropriately around me to get my attention, probably because she really wants a mother-figure in her life, but that is not the fault of anyone else, either, and it was still our vacation. I suggested BIL use the time without the son to do some bonding with the daughter, but he chose not to let his son come with us. When he told us, we then suggested that the 3 of them accompany us on a longer trip we have planned for the Fall, and he agreed.(Now I am having second thoughts, as he acts like a 9-year-old, but requires an adult pass and eats ALOT more!) As this child is not your sister's child(am I understanding this right, he is from a previous relationship?) and he is coming to live with your sister and BIL on a more permanent basis, taking the younger 2 children on a trip may give the boy a chance to adjust to his new residence without the interference of younger half-siblings(if I am correct in assuming that he is older than your niece and nephew). If you're close enough to your sister, just ask her what you should do...they may be unable to afford another pass and airfare, anyway, and I doubt she would expect you to pay for it. If you do end up taking him out of courtesy, I hope it is a pleasant experience for you. We recently(March) took my sister and 13-year-old nephew, who is an only child(for a few more months!) to WDW with us. I expected the worst of my nephew, and he delivered a little mischief, but in all fairness, my sister was more annoying than him! My children are still mocking her "discipline" tactics! Good luck with your decision.