Female family members in Medieval II: Total War include daughters of family members or wives from other families.
Daughters[]
Faction leaders in christian Factions daughters are Princesses. All other daughters stay available for random marriage events till age of 40(if they wont get married by that time they will get grey portrait and description Nun).
Wives[]
Most adult members will get random wife's from game events. Faction leader or faction heir may be married to other or same Faction princess. Both sexes are fertile till age of 50. Strangely, male members can marry till age of 75. Maximum age of both sexes are 105, but starting 60 they are considered old and have high chance of dying.
Strategy[]
Its best marry soon as possible, only keeping Faction leader or Faction heir for marriage with princesses.