In elliptical constructions, some words are understood but left out. When an elliptical construction ends in a pronoun, put the pronoun in the case it would be in if the construction were complete.

His sister has always been more athletic than he [is].

In some elliptical constructions, the case of the pronoun depends on the meaning intended.

Willie likes Lily more than she [likes Lily].

She is the subject of the omitted verb likes.

Willie likes Lily more than [he likes] her.

Her is the object of the omitted verb likes.