"My sister is interesting."
Translation:Mi hermana es interesante.
That is a good question. There are two verbs for "to be" in Spanish, ser and estar. "Es" is a conjugation of "ser" and "está" is a conjugation of "estar".
The rule of thumb is that Ser is used to describe qualities & attributes, as here "es interesante", whereas Estar is used to describe states, feelings, e.g. "I am tired"="Estoy cansado".
There are exceptions to the rule, for a full explanation see the link below or google DOCTOR PLACE. Hope this helps.