something belongs to someone. 

Michael and Elizabeth are iceskating.  His ice skates are black. [We use his because the possessor is Michael (third person masculine and singular)]. Her ice skates are green. [This time, we use her because the possessor is Elizabeth (third person feminine and singular)].

  a person is related to someone.

Elizabeth is angry with her son Peter.   [her indicates that Peter is related to Elizabeth].