None. Maybe it's worth to note that the relation between Maria Elena and Juan Antonio is full of aggression and pain, not a good example for anybody. Cristina's appearance solves their long-existing problems that may suggest that it's a good idea trying to solve a couple's struggle by involving a third. Which is almost always not true. Strong and thriving non-monogamous relationships - if they are reached by opening up an existing one - never come from weak and dying "basic" relationships.