Anybody from the same sex or those of opposite sex that have not passed puberty are considered as mahram.
A woman's opposite-sex mahrams fall into four categories (three categories in the strict-sense definition that does not count one's spouse). Note that mahrams for a man can be derived in a similar manner.
husband
permanent or blood mahrams with whom you become mahram by a blood relationship:
father, grandfather, great-grandfather and so on;
brother;
son, grandson, great-grandson;
uncle, parents' uncle, grandparents' uncle and so on;
nephew, grandnephew, great-grandnephew and so on;
in-law mahrams with whom you become mahram by marrying someone:
father-in-law;
son-in-law;
step-father (mother's husband) if their marriage is consummated;
step-son (husband's son) if her marriage is consummated;
radha' or milk-suckling mahrams with whom you become mahram because of being nursed by a woman. When a woman breast feeds an infant that is not her own child for a certain amount of time with certain conditions, she becomes the child's radha' mother and all said about blood mahrams applies here like radha' father/mother, radha' sister/brother, radha' aunt/uncle and so on.