Maternal relates to a mother — maternal instinct, or the maternal side of a family. Fraternal relates to brothers — fraternal bonds, a fraternal organisation — and, in the twin sense, means non-identical. They are not a pair despite sounding like one: the counterpart of maternal is paternal.
The core difference
These are frequently treated as opposites and are not. The genuine pairs run differently.
- maternal — of a mother: maternal grandmother (your mother's mother).
- paternal — of a father: paternal grandmother (your father's mother).
- fraternal — of brothers, or non-identical twins.
- sororal — of sisters, though rarely used.
Family-tree usage
Maternal and paternal are how English distinguishes which side of the family somebody is on, because it has no separate words for the two grandmothers — unlike many languages, which do.
The twin sense
Fraternal twins means non-identical, from two eggs, regardless of the twins' sexes. Two sisters can be fraternal twins, which surprises people. The technical term dizygotic avoids the mismatch.
