My sister is prettier than she. (Strictly speaking, this is the correct version)
BUT My sister is pretter than her. (Strictly to be used in speech only.)
However, according to some grammar books, 'My sister is pretter than her' is correct. I think modernEnglish favours this usage. I say so because this version is found in the newer grammar book
"When as and than are followed by personal pronouns, both subject and object forms are possible.
e.g. I think you understand the problem better than I / me
In informal English, object forms (me, her, him...) are much more common. Subject forms (I, she, he...) are more often used in a formal style, and some