Well, the topic is pretty deep and the examples you casted have a multitude of explanation in their own realms. I don't think it is possible for me to cast my view on all the perspectives in this comment.

Yes, everything happening in this world is wired with every other thing. Our actions DO make an impact on other people and objects in a certain way. Nevertheless, what impact our actions will have is not predictable by the actions alone. The traits of the recipient of that action have a great influences on what it makes out of that intake. For instance, the water is same for all but the seed decides if the plant will turn into an apple tree or a cactus.

So far as the example of love for the nieces is concerned..... yes, it happens that we unintentionally start showing appreciation for one more than the other because of their own traits and then the actions create ripple effect. Nevertheless, the situation does not ONLY boils down to the fight or competition between the two. For instance, if you only had Mahnoor (and not any other competitor) with the same kind of rebellion and independent personality she has, would you father be appreciative of her the same way he is of your other niece today?