El varón no puede unirse sexualmente ni casarse con su madre, con sus hermanas, primas, sobrinas o tías; tampoco la mujer puede unirse con su padre, con sus hermanos, primos, sobrinos o tíos. Quien es hermano, no es padre, ni hijo, ni sobrino ni esposo. El incesto conllevaría una confusión en las principales líneas de identidad personal y familiar, puesto que la hija o la hermana pasaría a ser esposa, etc. Se trata de proteger las relaciones surgidas en el interior de un sistema de parentesco, en virtud de esta ley que prohíbe el incesto. El matrimonio entre primos sí puede ser dispensado.

La dispensa o “relajación” de una ley meramente eclesiástica al caso particular, sólo puede ser concedida por quienes tienen competencia y potestad para hacerlo, ya sea por propio derecho o por legítima delegación.

http://www.am-abogados.com/blog/la-dispensa-canonica-de-los-...

In the jurisprudence of canon law of the Catholic Church, a dispensation is the exemption from the immediate obligation of law in certain cases.[1] Its object is to modify the hardship often arising from the rigorous application of general laws to particular cases, and its essence is to preserve the law by suspending its operation in such cases.

https://en.wikipedia.org/wiki/Dispensation_(canon_law)

In regard to their origin, impediments are either from divine law, and so cannot be dispensed, or from ecclesiastical law, and so can be dispensed by the competent Church authority. Under the 1983 Code of Canon Law, ecclesiastical impediments only apply to marriages where one or both of the parties is Catholic. Under the prior 1917 Code, ecclesiastical impediments applied to the marriages of non-Catholic Christians as well, unless specifically exempted. Note that, as clarified by articles 2 and 4 of Dignitas Connubii,[4] the Catholic Church now recognizes the diriment impediments of other (i.e., non-Catholic) Churches and ecclesial communities when their members are parties to a marriage.

https://en.wikipedia.org/wiki/Impediment_(canon_law)#Impedim...

DISPENSATIONS

Can. 85 A dispensation, or the relaxation of a merely ecclesiastical law in a particular case, can be granted by those who possess executive power within the limits of their competence, as well as by those who have the power to dispense explicitly or implicitly either by the law itself or by legitimate delegation.

http://www.vatican.va/archive/ENG1104/_PA.HTM