O.k. so here's the one piece of information I have to throw out into the "aunt/uncle" universe. If you are staying overnight with one family, having just returned from a trip, do not pull out the gifts you have bought for your other sibling's children to show everyone, and have absolutely nothing for the sibling's child who lives in the house where you are staying! My dh's older sister came back from the UK, and stayed at our house overnight, since she didn't want to drive all the way home. We were happy to have her as a guest, until she proceeded to get out all the items she bought for my nieces and show my son. We had opened our home to her (not to mention providing airport shuttle service and garaging her car while she was away), and she didn't even have the presence of mind to bring my son anything at all. Seriously, his face would not have fell (being 4 and all), if she had picked up a pen at the airport! I respect her right to buy for whom she wants to when she wants to, but that was too much!
Off my soapbox -- the gist is, if you are blatantly treating them differently, and they are going to know about it, think about it...