Objective Case or Accusative Case
Grammar Index : 2I met your sister.
Noun or pronoun is said to be in Objective Case or Accusative Case if it is the direct object of verb or if it is the object of preposition. (Direct object is the person or the thing upon whom or upon which the action of the verb is carried out).
Your sister" is in objective case.
The vendors sell mangoes.
Mangoes" is in objective case.
The book is on the table.
Table" is in objective case.
It is object of the preposition ‘on’.