Hi Carolina - It is grammatically incorrect to use 'My sister and me went to the theatre' because 'me' is not a subject pronoun (it's an object pronoun), and I think it's easier to understand if you remove one of the subjects - you wouldn't say 'me went to the theatre last week'. However, you could ask 'Did you get theatre tickets for me (and my sister)?' Hope this helps.

"My sister and I" is the correct way to say it, because I is a subject pronoun, however you will hear even native speakers use "me" instead of I. There's no difference in meaning.