Love is more of a decision than a feeling. Even in the absence of fuzzy feelings, it is possible to make the decision to treat a child well. I doubt that many parents have absolutely equal feeling for all their children. I don't think your emotional response is something that you can control, as you've found out yourself. Some kids are just the apple of a parent's eye and some are not.

You've made a decision to withhold appropriate treatment of your eldest child. Decisions can be changed. At some level you are punishing her for something. Maybe someone was mean to you as a child. Perhaps this is making you feel powerful. Withholding from someone is very much a power play. At some level, you may have sociopathic tendencies. It seems you base how you interact with your children on your own emotional gratification. You do what gratifies you with the second. You don't bother with the first if there is not emotional pay off for you. Part of maturity is responding to children based on their needs, rather than your own.

What you're engaging in is a subtle form of mental torture. Why anyone would find satisfaction in doing that is not something that I or any professional counselor could explain. But it is a fact of human nature that many human beings take perverse satisfaction in inflicting emotional pain. It's one of the mysteries of life why that is.