Mahram Aur Na Mahram Concept In Islam
See the list in the verses 4:22 to 24. https://www.javedahmedghamidi.org/#!/quran?chapter=4&paragraph=15&type=Ghamidi
Mehram means the one with whom you can marry. There are two kinds of mehram. Permanent mehram, close blood relations like mother, sister, daughter, in laws, mother in law, daughter in law. Foster relations, like foster mother, foster sister. Temporary mehram like sister in law.