I've been pondering on this all afternoon. I love my children equally, as in I don't have any favourites, but I have always treated them differently because they have very different personalities. I never keep track of how much we spend on each child so I have no idea of the discrepancy in spending on each of them, but the important thing is that they really don't keep track either.
Similarly, I have recently spent a lot of money on my daughter and new granddaughter and on flights to visit them in NZ, but my other daughter wouldn't dream of querying whether I spent more on my granddaughter than I did/do on her sons. When I buy Christmas presents for my grandsons I tend to buy the same number of gifts for each, but the prices may vary enormously. The children don't know how much things cost and the rest of the family don't care. I choose the present I think they'd like and that I want to buy.
In my birth family however I can see that some of my siblings would probably be bothered if my mum gave to one and not the other. Nothing to do with their finances and everything to do with not feeling 'left out' or treated unfairly.