A mahram is a person with whom sex is forbidden forever due to kinship. Either because of blood ties, milk kinship or affinity.
Wikepedia States -
In Islamic sharia legal terminology, a mahram (Arabic محرم, also transliterated mahrim or maharem) is an unmarriageable kin with whom sexual intercourse would be considered incestuous, a punishable taboo. Current usage of the term covers a wider range of people and mostly deals with the dress code practice of hijab.