Is it true that parents favour one child over another? Does this mean that their parents love them less?

We are three sisters and one brother and though my mother was always partial to me being the youngest, she was never unfair to my other siblings and we never had a problem whatsoever. Parents normally are more sensitive to the needs of a weak child or may be one that needs more protection, but that does not mean that they don't love their other kids. For a mother all kids are equal and at the bottom of her heart she cares for all of them.