Student or Learner
1) Since my sister does not speak english, so she will get somebody to call you on her behalf with my sister besides the person at the time when the call is made.
not at teacher
Since my sister does not speak English, she will speak to you through an interpreter on the phone.
An interpreter only translates what the two people say. The interpreter will not say anything else, except to have both parties know that (s)he is the interpreter.
In the sentence in the original post, somebody is actually making the call for the sister. That somebody knows what the sister wants to say and will say it on behalf of the sister. The sister will be there in case something comes up that the caller doesn't know how to reply to, and the caller can then ask the sister what the caller should say.
That is what her friend is essentially doing - to help translate the message from one language to another i.e. performing the function of an interpreter. The sentence does not say the person who is doing the translation would say more than that. 'Interpreter' may sound a bit official but you can have an friend to be your interpreter, can't you? It could be the friend or the sister making the call but that is not the point and obviously both have to be present. My idea is just to cut short a long-winded sentence.
But I feel very strongly that we must not change the meaning. If we change the meaning, we will confuse the person who is seeking our help. I think we should just correct the mistakes in the sentence. Otherwise, the writer will not know what was good and bad about (his)(her) sentence.