I know, I know..my mom says it too, she doesn’t have favorites, she loves us all the same.   Actually, I have come to recognize that I don’t want to be loved ‘the same’ as my siblings, because somehow it denudes my specialness, and I am then just one of the brood.  My mother would never admit to having a favorite among her four children.  I like to believe that I don’t have a favorite among my three children.  Yet this article in the New York Times; Mom Always Like You Best (a well known routine from the Smothers Brothers) points out the significance of  who is going to care for mom and how that relates to mom’s sense of  her favorite child.
Quoting from the article, and for more, read the article here.

Further studies revealed that middle-aged children often recognized that their parents felt closer to one child than another — but were off-base about who ranked highest. “They typically choose themselves,” Dr. Pillemer said, “and they’re typically wrong.”
One might file this under “Stuff I’d Just as Soon Not Know,” except that the care of the elderly falls mostly to their children and that one child usually shoulders the bulk of the responsibility. Mothers also express clear ideas about whom they want and expect to take on that role, it turns out, so their partiality has consequences.