If you want to say with your sister in German would it be mit meine Schwester or mit meiner Schwester?

Answer 1

I believe that mit takes the genitive case, which for the definite articles are:

dem der dem den

(masc) (fem) (neut) (plur)

So if you were talking about your sister as the subject, that would be the nominative case: die Schwester, making it meine Schwester.

But if you were talking about something with your sister, you would use der Schwester making it mit meiner Schwester. Remember, you would use the uns conjugation of the verb, too, if you are part of the pair or group, so tanzen, essen, ins Kino zu Film angesehen gegangen (oder was du willst).