Sadly some parents make it very obvious that they favour one child over another and this is distressing for all concerned - except perhaps the favourite child! And does nothing to alleviate sibling rivalry.
While it is perfectly natural to find yourself more in tune with one child from time to time, you should ensure that you treat them equally within a framework that is fair and even-handed - fairness in bedtimes, having friends over, pocket money and so on.
Be assured that the one you favour now may change in time - in fact invariably does and the situation is not necessarily enduring. It is common to find yourself more attracted to one child. Just because you are biologically related doesn't mean that you are equally attracted to all.
Each brother's or sister’s relationship to their mother is different because of their non-shared environment - they are genetically but not environmentally related. Different temperaments, genders, birth-places and birth order all contribute to how you see a child. One child may remind you of someone or you may see yourself in them. If you dislike something in yourself you are likely to criticise your child if she shows the same tendency!
If you find yourself liking one child more than another and are worried about it, talk to someone about your feelings. Don’t pretend it’s not happening; discuss it with a co-parent, a friend, health visitor or someone else you trust. Get reassurance.
Try to find the basis for your feelings. Is one child difficult to get on with? Being aware of this is immediately helpful. Try to put more energy into an interest you can all enjoy. Shared activities bring a wonderful opportunity to talk spontaneously and this has all sorts of beneficial knock on effects. You may even find the child you thought you had nothing in common with shares or develops interests akin to your own!