In her novel, Laura Esquivel uses a fictional style called "magic realism", a technique found frequently in Latin American Literature. In magic realism, elements of the fantastic are intermingled freely with the realistic. Tita, caring for her baby nephew when his wet-nurse is mortally wounded, is at her wits' end when the frantic baby refuses to take cow's milk or tea. Out of desperation she offers the child her own breast, thinking it would "at least act as a pacifier and keep him occupied while she decided what to do to appease his hunger". She is shocked when she finds she is able to actually feed the baby - it is nothing short of "a supernatural act" (Chapter 4). The magic of this particular situation is one of many which Esquivel uses to reflect of one of the main themes of the book - the intense emotion created by the love between Tita and Pedro which cannot be thwarted.
Human lactation works primarly on a supply and demand system. A woman who has never been pregnant or given birth can develop a milk supply through several processes, the simplest of which would be putting the infant to her own breast regularly. Infant sucking provides a specific type of stimulation that signals the milk glands to begin production. In emergency situations, women long past childbearing or nonparous women have suckled infants and developed milk supplies adequate to maintain the infant's health.Typically, the process of adoptive breastfeeding is more time consuming, but it is certainly possible. While elements of magical realism come into play in this scenario, through the speed at which Tita develops a milk supply adequate enough to sustain an infant, the actual biological mechanics of breastfeeding are such that the act of adoptive breastfeeding is completely possible.
In Like Water For Chocolate the reason that Tita is feeding her nephew is because when Roberto was born his mother run out of her own breast milk so then Tita have steps in and offer herself to feeds her nephew. She coul not bare to see her nephew of hunger. She love her nephew and she always thought what ever happened ti him she will not forgive herself. You said that she gets pregnant towards the end of the book, but that is no neccessary true because she is no. She thoughtthat she was pregnant I think but it turns out to be no.