No, it's not a proper English phrase. 'Suck soms cock' is an inappropriate and ungrammatical construction. The whole phrase seems very odd and not in line with standard English usage.
No, it is completely unethical. Incestuous relationships are not acceptable in most societies around the world. It goes against moral, ethical, and often legal norms. Family relationships are meant to be based on love, respect, and appropriate boundaries, not sexual relations.
Most mainstream movies avoid such explicit and disturbing content. The movie industry generally adheres to certain moral and ethical standards, and such a plot would be considered inappropriate and offensive for a wide audience. However, in some very low - budget or underground movies that are not subject to the usual regulations, there might be some extremely rare and inappropriate depictions, but these are not the norm.
Such stories often involve very disturbing and unethical content. However, in a civilized and legal society, prisons are supposed to be places of rehabilitation and protection of rights. While there may be individual untrue rumors or fictional accounts, the vast majority of prisons have strict regulations to prevent any form of sexual abuse.