Musar על תענית 13:5

The gist of the story is that cause and effect are described as feeding on each other. The effect, i.e. עלול, is "beneath" i.e. the result of the עילה, "the cause." This is why it is described as בת, daughter. The first level of achievement in a developing and deepening relationship is called "daughter." When this relationship develops further, the result at the next level is described as "sister." When the relationship intensifies still more the result is called "my mother." This process can be compared to an infant suckling on the mother's breast. The very suckling increases the mother's supply of milk, i.e. the infant itself is not only the effect but also the cause of the mother being a true mother. Our sages in Moed Katan 16 have G–d ask: מי מושל בי? "Who governs My actions?" and answer: צדיק, שאני גוזר גזרה והוא מבטל אותה, "the righteous, for I make a decree and he annuls it." A statement in a similar vein is found in Taanit 7a where Rabbi Chanina says: "I have learned a great deal from my teachers; I have learned even more from my colleagues; the most I have learned from my students." The students mentioned there are analogous to the "daughter" mentioned in the Midrash Chazit above. The colleagues are analogous to the "sister, and the "mother" mentioned there is analogous to the "teacher," i.e. רב.