I'm surprised to discover that there does not seems to be an equivalent in Spanish for the English word Mean, as in: She is always so mean to her sister.
None of the translations I've found really catch the English word.
She is always so mean to her sister = Siempre es tan mala con su hermana
my chilean friends have told me that "pesada" is mean, but I am pretty sure that it's a slang word...
You could use the word "Malo" it really is more less the exact same thing wen you really think about it.
Siempre trata mal y con deprecio a su hermana.