Pedro's flaws are many. First, he falls instantly in love with Tita, showing that he is superficial. His love turns out to be true, but then he chooses to marry Rosaura (Tita's sister) under the premise of wanting to be close to Tita.
His actions affect Tita and Rosaura the most. Tita is crushed when he chooses to marry her sister and spends a lifetime wishing that they could be together. She is caught in an affair with Pedro for her entire life and it isn't until the end of the novel that they actually get to be together.
Rosaura is forced into a marriage with Pedro, a man she knows does not love her at all and is madly in love with her sister. This leads her to be bitter and mean towards Tita and affects the dynamic of the entire household, especially considering the fact that Tita is essentially their "servant". Tita also becomes pregnant with Pedro's child, further the complications. When Rosaura cannot nurse her and Pedro's child and Tita steps in, the tension and anger reach a boiling point.
At the heart of all the conflict is Pedro. His decision to marry Rosaura despite being in love with Tita changes everything.